ABINIT, GW input variables:
List and description.
This document lists and provides the description of the name (keywords) of the GW input variables to be used in the main input file of the abinit code.
The new user is advised to read first the new user's guide, then the abinit help file, before reading the present file. It will be easier to discover the present file with the help of the two lessons lesson_gw1 and lesson_gw2 of the tutorial. Other input variables directly related to the GW computation are : getkss getscr optdriver. In case of parallel execution, the input variable gwpara is used to choose between the different parallelization levels presently implemented (k-points or bands).
Note also the Mrgscr utility which allows one to merge dielectric matrices calculated at different q-points into a unique screening file that can be used for subsequent GW calculations.
Copyright (C) 2002-2011 ABINIT group (XG)
This file is distributed under the terms of the GNU General Public License, see
~abinit/COPYING or
http://www.gnu.org/copyleft/gpl.txt .
For the initials of contributors, see ~abinit/doc/developers/contributors.txt .
Goto : ABINIT home Page | Suggested acknowledgments | List of input variables | Tutorial home page | Bibliography
Help files : New user's guide | Abinit (main) | Abinit (respfn) | Mrgddb | Anaddb | AIM (Bader) | Cut3D | Optic
Files that describe other input variables:
- Basic variables, VARBAS
- Developper variables, VARDEV
- File handling variables, VARFIL
- Geometry builder + symmetry related variables, VARGEO
- Ground-state calculation variables, VARGS
- Internal variables, VARINT
- Parallelisation variables, VARPAR
- Projector-Augmented Wave variables, VARPAW
- Response Function variables, VARRF
- Structural optimization variables, VARRLX
- Wannier90 interface variables, VARW90
Content of the file : alphabetical list of variables.
A. awtr
B. bdgw bs_algorithm bs_haydock_niter bs_hayd_term bs_haydock_tol bs_loband bs_eh_cutoff bs_exchange_term bs_coulomb_term bs_calctype bs_coupling bs_freq_mesh
C. cd_custom_imfrqs cd_full_grid cd_halfway_freq cd_imfrqs cd_max_freq cd_subset_freq cd_use_tangrid chkgwcomp
D.
E. ecuteps ecutsigx ecutwfn
F. fftgw freqremax freqremin freqspmax
G. gwcalctyp gwcomp gwgamma gwmem gwrpacorr gw_eet gw_eet_inclvkb gw_eet_nband gw_eet_scale gw_nstep gw_nqlwl gw_qlwl gw_sctype gw_sigxcore gw_toldfeig
H.
I. inclvkb icutcoul
J.
K. kptgw
M.
N. nbandkss nfreqim nfreqmidm nfreqre nfreqsp npwkss nkptgw nomegasrd nomegasi nomegasf npweps npwsigx npwwfn nsheps nshsigx nshwfn
O. omegasimax omegasrdmax
P. ppmfrq ppmodel
Q. qptdm
R. rhoqpmix
S. soenergy spbroad spmeth symchi symsigma
T.
U.
V.
W.
X.
Y.
Z. zcut
awtr
Mnemonics: evaluate the Adler-Wiser expression of $\chi^{(0)}_{KS}$ assuming Time-Reversal
Characteristic: GW
Variable type: integer parameter
Default 1 (was 0 prior to 5.8)
Only relevant if optdriver=3, that is, screening calculations.
This input variable defines whether the irreducible polarizability $\chi^{(0)}_{KS}$ is evaluated taking advantage of time-reversal symmetry or not.
- 0 => Use the "standard" Adler-Wiser expression without assuming time-reversal symmetry. In this case, the irreducible polarizability is calculated summing over all possible electronic transitions (both resonant and antiresonant).
- 1 => Take advantage of time-reversal symmetry to halve the number of transitions to be explicitly considered. This method leads to a decrease in the CPU time by a factor two with respect to the awtr=0 case.
Note that the parallel algorithm gwpara=2 is not compatible with
the choiche awtr=0.
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bdgw
Mnemonics: BanDs for GW calculation
Characteristic: GW
Variable type: integer array bdgw(2,nkptgw, nsppol,)
Default is all 0's
Only relevant if optdriver=4, that is, sigma calculations.
For each k-point with number ikptgw in the range (1:nkptgw) and each spin index isppol, bdgw(1,ikptgw,isppol) is the number of the lowest band for which the GW computation must be done, and bdgw(2,ikptgw,isppol) is the number of the highest band for which the GW computation must be done.
When gwcalctyp >= 20, the quasiparticle wavefunctions are computed and represented as linear combination of some Kohn-Sham wavefunctions. In this case bdgw designates the KS wavefunctions used as basis set. For each k-point, indeed, the quasiparticle wavefunctions are expanded considering only the KS states between bdgw(1,ikptgw,isppol) and bdgw(2,ikptgw,isppol).
Note that the initial values given in the input file might be changed inside the code so that all the degenerate states at a given k-point and spin are included. This might happen when symsigma=1 is used or in the case of self-consistent GW calculations.
When symsigma=1, indeed, the diagonal matrix elements of the self-energy are obtained by averaging the unsymmetrized results in the degenerate subspace.
For self-consistent calculations, on the other hand, the basis set used to expand the GW
wavefunctions should include all the degenerate states belonging to the same irreducible
representation. Only in this case, indeed, the initial symmetries and energy degenerations are preserved.
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bs_algorithm
Mnemonics: Bethe-Salpeter ALGORITHM
Characteristic: BS
Variable type: integer
Default is 1
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
The bs_algorithm input variable defines the algorithm employed to calculate the macroscopic dielectric function. Possible values are 1, 2 or 3:
- 1 => The macroscopic dielectric is obtained by performing a direct diagonalization of the exciton Hamiltonian. Advantages: It gives direct access to the excitonic eigenvalues as well as to the oscillator strengths. Drawbacks: It is a very CPU- and memory-consuming approach as the size of the Hamiltonian scales as (nk*nc*nv)**2. where nk is the number of k-point in the FULL Brillouin zone, and nc and nv are the number of conduction and valence states, respectively. Pros: It can be used both for resonant-only and resonant+coupling calculations.
- 2 => Haydock iterative method. The macroscopic dielectric function is obtained by iterative applications of the Hamiltonian on a set of vectors in the electron-hole space. Advantages: It is less memory demanding and usually faster than the direct diagonalization provided that zcut is larger than the typical energy spacing of the eigenvalues. Drawbacks: It's an iterative method therefore the convergence with respect to bs_haydock_niter should be checked. It is not possible to have direct information on the exciton spectrum, oscillator strengths and excitonic wave functions. For the time being bs_algorithm=2 cannot be used for calculations in which the coupling term is included.
- 3 => Conjugate-gradient method [STILL UNDER DEVELOPMENT] Only available for resonant calculations.
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bs_haydock_niter
Mnemonics: Bethe-Salpeter HAYDOCK Number of Iterations
Characteristic: BS
Variable type: integer
Default is 100
Only relevant if optdriver=99 and bs_algorithm=2 that is, Bethe-Salpeter calculations with the Haydock iterative method.
bs_haydock_niter defines the maximum number of iterations used to calculate the macroscopic dielectric function.
The iterative algorithm stops when the difference between two consecutive evaluations of the optical
spectra is less than bs_haydock_tol.
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bs_hayd_term
Mnemonics: Bethe-Salpeter HAYdock TERMinator
Characteristic: BS
Variable type: integer
Default is 1
Only relevant if optdriver=99 and bs_algorithm=2 that is, Bethe-Salpeter calculations with the Haydock iterative method.
Defines how to terminate the continued fraction expression for the dielectic function. The terminator reduces the number of iterations needed to converge by smoothing the oscillation in the high energy part of the spectrum
- 0 => No terminator. The contribution given by the terms missing in the Lanczos chain are set to zero.
- 1 => Use the terminator function. The particular expression depends on the type of calculation: In the resonant-only case, the a_i and b_i coefficients for i > niter, are replaced by their values at i=niter. Ehen the coupling block is included, the terminator function described in D. Rocca, R. Gebauer, Y. Saad, S. Baroni, J. Chem. Phys. 128, 154105 (2008) is used.
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bs_haydock_tol
Mnemonics: Bethe-Salpeter HAYDOCK TOLerance
Characteristic: BS
Variable type: real(2)
Default is (0.02,0)
Only relevant if optdriver=99 and bs_algorithm=2 that is, Bethe-Salpeter calculations with the Haydock iterative method.
Defines the convergence criterion for the Haydock iterative method. The iterative algorithm stops when the difference between two consecutive evaluations of the macroscopic dielectric function is less than bs_haydock_tol(1). The sign of bs_haydock_tol(1) defines how to estimate the convergence error. A negative value signals that the converge should be reached for each frequency (strict criterion), while a positive value indicates that the converge error is estimated by averaging over the entire frequency range (mild criterion).
bs_haydock_tol(2) defines the quantity that will be checked for convergence:
- 0 → both the real and the imaginary part must converge
- 1 → only the real part
- 2 → only the imaginary part
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bs_loband
Mnemonics: Bethe-Salpeter Lowest Occupied BAND
Characteristic: BS
Variable type: integer
Default is 0
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
This variable defines the index of the lowest occupied band used for the construction of the electron-hole basis set
An additional cutoff energy can be applied by means of the bs_eh_window input variable.
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bs_eh_cutoff
Mnemonics: Bethe-Salpeter Electron-Hole CUTOFF
Characteristic: BS
Variable type: integer(3)
Default is (-inf,+inf,+inf)
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
It is used to define a cutoff in the e-h basis set. Only those transitions whose
energy is between bs_eh_window(1) and bs_eh_window(2) will be considered during
the construction of the e-h Hamiltonian. bs_eh_window(3) (named stripecut in EXC)
are used to define an additional cutoff in the exciton Hamiltonian. Let it and itp
indicate two e-h basis set elements with itp >= it.Only the matrix elements
corresponding to the pairs (it,itp) such that Etrans_{itp}-Etrans_{it} are considered.
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bs_exchange_term
Mnemonics: Bethe-Salpeter EXCHANGE TERM
Characteristic: BS
Variable type: integer
Default is 1
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
- 0 => The exchange term is not calculated. This is equivalent to neglecting local field effects in the macroscopic dielectric function.
- 1 => The exchange term is calculated and added to the excitonic Hamiltonian.
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bs_coulomb_term
Mnemonics: Bethe-Salpeter COULOMB TERM
Characteristic: BS
Variable type: integer
Default is 11
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations. This variable governs the choice among the different options that are available for the treatment of Coulomb term of the BS Hamiltonian. bs_coulomb_term is the concatenation of two digits, labelled (A) and (B).
The first digit (A) can assume the values 0,1,2:
- 0 => The Coulomb term is not computed. This choice is equivalent to computing the RPA spectrum but using the representation in transition space instead of the more efficient approach based on the sum over states.
- 1 => The Coulomb term is computed using the screened interaction read from an external SCR file (standard excitonic calculation).
- 2 => The Coulomb term is computed using a model screening function (useful for convergence studies or for reproducing published results).
The second digit (B) can assume the values: 0,1
- 0 => Use a diagonal approximation for W_GG' (mainly used for accelerating convergence studies).
- 1 => The Coulomb term is correctly evaluated using the truly non-local W(r,r').
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bs_calctype
Mnemonics: Bethe-Salpeter CALCulation TYPE
Characteristic: BS
Variable type: integer
Default is 1
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
Possible values are 1,2,3
- 1 => use the KS eigenvalues and wave functions stored in the KSS file to construct the transition space
- 2 => The transition space is constructed with Kohn-Sham orbitals but the energies are read from the external GW file
- 3 => QP amplitudes and energies will be read from the QPS file and used to construct H_ex. Not coded yet Besides <\psi|r|\psj>^QP should be calculated taking into account the non-locality of the the self-energy in the commutator [H,r].
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bs_coupling
Mnemonics: Bethe-Salpeter COUPLING
Characteristic: BS
Variable type: integer
Default is 1
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
The bs_coupling input variable defines the treatment of the coupling block of the BS Hamiltonian. Possible values are 0,1.
- 0 => The coupling block is neglected. The code runs faster and the Hamiltonian matrix requires less memory (factor 4). It is a good approximation for the absorption spectrum which only requires the knowledge of Im(\epsilon). The reliability of this approximation should be tested in the case of EELF calculations.
- 1 => The coupling term is included.
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bs_freq_mesh
Mnemonics: Bethe-Salpeter FREQuency MESH
Characteristic: BS, ENERGY
Variable type: real(3)
Default is (0.0,0.0,0.01) eV
Only relevant if optdriver=99, that is, Bethe-Salpeter calculations.
bs_freq_mesh(1) defines the first frequency for the calculation of the macroscopic dielectric function.
bs_freq_mesh(2) gives the last frequency for the calculation of the macroscopic dielectric function.
If zero, bs_omegae is automatically calculated inside the code such that the frequency mesh covers the entire set of e-h transitions.
bs_freq_mesh(3) gives the step of the linear mesh used for evaluating the macroscopic dielectric function.
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bs_nstates
Mnemonics: Bethe-Salpeter Number of States
Characteristic: BS
Variable type: integer
Default is 0
Only relevant if optdriver=99 and bs_algorithm=2,3 that is, Bethe-Salpeter calculations with the Haydock iterative approach or the Conjugate gradient method
bs_nstates defines the maximum number of excitonic states calculated in
the direct diagonalization of the excitonic matrix or in the conjugate-gradient method.
The number of states should be sufficiently large
for a correct description of the optical properties in the frequency range of interest.
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cd_custom_imfrqs
Mnemonics: Contour Deformation Custom Imaginary Frequencies
Characteristic: GW
Variable type: integer
Default is 0 (off)
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations, and gwcalctyp=2, 12 or 22.
cd_custom_imfrqs lets the user define the gridpoints along the imaginary axis by hand. Set this
to the number of frequencies you want. The frequencies are specified with cd_imfrqs.
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cd_imfrqs
Mnemonics: Contour Deformation Imaginary Frequencies
Characteristic: GW
Variable type: real array cd_imfrqs(cd_custom_imfrqs)
Default is none
Only relevant if optdriver=3, that is, screening calculations, and gwcalctyp=2, 12 or 22.
cd_imfrqs Specifies the grid points for the imginary axis. Only activated if cd_custom_imfrqs is not equal to 0. The number of frequencies is set by the value of cd_custom_imfrqs. Example:
cd_custom_imfrqs 5 nfreqim 5 cd_imfrqs 0.1 0.2 0.5 1.0 5.0If nfreqim is not equal to cd_custom_imfrqs a warning will be issued.
Use at own risk! The use of a custom grid is very likely to disqualify your SUS and SCR file from use in self-energy (i.e. optdriver=4) calculations, unless the gridpoints are picked carefully. Note that frequencies have to be strictly increasing.
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cd_full_grid
Mnemonics: Contour Deformation Full Grid in complex plane
Characteristic: GW
Variable type: integer
Default is 0 (off)
Only relevant if optdriver=3, that is, screening calculations, and gwcalctyp=2, 12 or 22.
cd_full_grid enables the calculation of the screening [both chi0 and epsilon^(-1)] on a grid in the first quadrant of the complex plane. The grid is determined by the (tensor) product of the grid in real frequency and the grid in imaginary frequency. In the SUS and SCR files the grid points are stored as follows:
Index: 1 . . . nfreqre nfrqre+1 . . . nfreqre+nfreqim nfreqre+nfreqim+1 . . . nfreqre*nfreqim Entry: | purely real freq. | purely imaginary freq. | gridpoints in complex plane |The grid in the complex plane is stored looping over the real dimension as the inner loop and the imaginary as the outer loop. The contents of the generated SUS and SCR files can be extracted for visualisation and further analysis with the mrgscr utility.
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cd_halfway_freq
Mnemonics: Contour Deformation tangent grid Halfway Frequency
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 100.0 eV
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations, and gwcalctyp=2, 12 or 22.
cd_halfway_freq determines the frequency where half of the number of points defined in
nfreqre are used up. The tangent transformed grid is approximately
linear up to this point. To be used in conjunction with cd_use_tangrid.
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cd_max_freq
Mnemonics: Contour Deformation grid Maximum Frequency
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 1000.0 eV
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations, and gwcalctyp=2, 12 or 22.
cd_max_freq determines the frequency where all the points defined in
nfreqre are used up. To be used in conjunction with
cd_use_tangrid.
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cd_subset_freq
Mnemonics: Contour Deformation grid calculate Subset of Frequencies
Characteristic: GW
Variable type: integer array cd_subset_freq(2)
Default is full set
Only relevant if optdriver=3, that is, screening calculations, and gwcalctyp=2, 12 or 22.
cd_subset_freq Specifies that only a subset of the frequencies defined by nfreqre are to be calculated. The first index is the start and the second the end, with index number 1 always being the origin. For example a calculation with nfreqre=100 could be separated into two datasets with:
subset_freq1 1 50 subset_freq2 51 100Any resulting susceptibiltiy (_SUS) and screening (_SCR) files can then be merged with the mrgscr utility.
Note that this does NOT
have to be used in conjunction with cd_use_tangrid.
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cd_use_tangrid
Mnemonics: Contour Deformation Use Tangent Grid
Characteristic: GW
Variable type: integer
Default is 0 (off)
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations, and gwcalctyp=2, 12 or 22.
cd_use_tangrid defines a nonuniform grid to be used in frequency, with stepsize increasing
proportional to tan(x). This makes the grid approximately linear to start with, with a rapid increase
towards the end. Also, this is the grid which gives equal importance to each point used in the integration
of a function which decays as 1/x^2. To be used in conjuction with nfreqre,
cd max_freq and cd halfway_freq
which determine the parameters of the transformed grid.
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chkgwcomp
Mnemonics: Check GW Completeness
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=3, that is, screening calculations.
chkgwcomp performs a check on the completeness relation that is assumend to be fulfilled by the oscillator strenghts
in the limit of very large number of bands.
If chkgwcomp==1, a _DELI file is generated, containing the incompleteness matrix in reciprocal space for each q-point.
This matrix is the difference between the identity and the projection over all bands (the oscillator strengths),
summed over all occupied states. It is relevant in case one performs a GW + PAW calculation, especially if one is
willing to use the extrapolar approximation (gwencomp=1).
Note that this closure relation is not fulfilled in PAW, due to the incompleteness of the basis.
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ecuteps
Mnemonics: Energy CUT-off for EPSilon (the dielectric matrix)
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
ecuteps determines the cut-off energy of the planewave set used to represent the
independent-particle susceptibility $\chi^{(0)}_{KS}$, the dielectric matrix $\epsilon$, and its inverse.
It is not worth to take ecuteps bigger than four times ecutwfn,
this latter limit corresponding to the highest Fourier components of a wavefunction convoluted with itself.
Usually, even twice the value of ecutwfn might overkill. A value of ecuteps
between 5 and 10 Hartree often leads to converged results (at the level of 0.01 eV for the energy gap).
In any case, a convergence study is worth.
This set of planewaves can also be determined by the other input variables
npweps and nsheps,
but these are much less convenient to use for general systems, than the selection
criterion based on a cut-off energy.
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ecutsigx
Mnemonics: Energy CUT-off for SIGma eXchange
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0 .
Only relevant if optdriver=4, that is, sigma calculations.
ecutsigx determines the cut-off energy of the planewave set used to generate the exchange part of the self-energy operator. For norm-conserving calculations, it is pointless to have ecutsigx bigger than 4*ecut, while for PAW calculations, the maximal useful value is pawecutdg. Thus, if you do not care about CPU time, please use these values. If you want to spare some CPU time, you might try to use a value between ecut and these upper limits.
This set of planewaves can also be determined by the other input variables
npwsigx and nshsigx,
but these are much less convenient to use for general systems, than the
selection criterion based on the cut-off energy (ecutsigx has to be 0.0 for using these).
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ecutwfn
Mnemonics: Energy CUT-off for WaveFunctions
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0, although, if a screening or self-energy calculation is done (optdriver=3 or 4), ecutwfn is automatically set to ecut
Only relevant if optdriver=3 or 4, that is, screening and sigma calculations.
ecutwfn determines the cut-off energy of the planewave set used to represent the wavefunctions
in the formula that generates the independent-particle susceptibility $\chi^{(0)}_{KS}$
(for optdriver=3), or the
self-energy (for optdriver=4).
Usually, ecutwfn is smaller than ecut,
so that the wavefunctions are filtered, and some components are ignored.
As a side effect, the wavefunctions are no more normalized, and also, no more orthogonal.
Also, the set of plane waves can be much smaller for optdriver=3,
than for optdriver=4, although a convergence
study is needed to choose correctly both values.
This set of planewaves can also be determined by the other input variables
npwwfn and nshwfn,
but these are much less convenient to use for general systems, than the
selection criterion based on the cut-off energy (ecutwfn has to be set to 0.0 for using these).
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fftgw
Mnemonics: FFT for GW calculation
Characteristic: GW
Variable type: integer parameter
Default is 21.
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
The basic ingredients needed to perform both a screening and a sigma calculation are the so-called
oscillator matrix elements defined as
$<
k-q, b1 | e^{-i (q+G).r} | k b2 >$ 1)
In reciprocal space, Eq.1 is given by a convolution in which the number of reciprocal
lattice vectors employed to describe the wavefunctions is given
by ecutwfn
Tn the case of screening calculations, the number of G vectors in Eq.1 is defined
by ecuteps,
while ecutsigx defined the number of G used in sigma calculations.
To improve the efficiency of the code, the oscillator matrix elements are evaluated
in real space through FFT techniques, and the fftgw input variable is used to select the FFT
mesh to be used.
fftgw is the concatenation of two digits, labelled (A) and (B) whose value is internally used to define the value of ngfft(1:3) (see the setmesh.F90 routine).
The first digit (A) defines the augmentation of the FFT grid. Possible values are 1, 2 and 3.
- 0 => Use the FFT grid specified by the user through ngfft(1:3)
- 1 => Use a coarse FFT grid which encloses a sphere in reciprocal space whose radius depends on the largest value between ecutwfn and ecuteps
- 2 => Use a slightly augmented FFT which is sufficient for the correct treatment of the convolution
- 3 => Doubled FFT grid (same mesh as that used for GS calculations).
The second digit (B) can be chosen between 0 and 1. It defines whether a FFT grid compatible with all the symmetries of the space group must be enforced or not:
- 0 => Use the smallest FFT mesh which is compatible with the FFT library (faster, save memory but is less accurate)
- 1 => Enforce a FFT grid which is compatible with all the symmetry operations of the space group. This method leads to an increase both of CPU time and memory, but the matrix elements are more accurate since the symmetry properties of the system are preserved.
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freqremax
Mnemonics: FREQuencies along the Real axis MAXimum
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0
Only relevant if optdriver=3, that is, screening calculations.
freqremax is used only for numerical integration of the GW self-energy
(gwcalctyp= 2, 12, 22, 9, 19, 29).
freqremax sets the maximum real frequency used to calculate the dielectric matrix in order
to perform the numerical integration of the GW self-energy.
freqremax, freqremin and nfreqre
define the spacing of the frequency mesh along the real axis.
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freqremin
Mnemonics: FREQuencies along the Real axis MINimum
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0
Only relevant if optdriver=3, that is, screening calculations.
freqremin is used only for numerical integration of the GW self-energy
(gwcalctyp= 2, 12, 22, 9, 19, 29).
freqremin sets the minimum real frequency used to calculate the dielectric matrix in order
to perform the numerical integration of the GW self-energy.
freqremin can be used to split a wide frequency interval into smaller subintervals that
can be calculated independently.
The different subintervals can then be merged together with the Mrgscr utility thus obtaining
a single screening file that can used for self-energy calculations.
Note that freqremin, freqremin
and nfreqre define the spacing of the frequency mesh along the real axis.
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freqspmax
Mnemonics: FREQuencies for the SPectral function MAXimum
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0
Only relevant if optdriver=4, that is, sigma calculations.
freqspmax sets the maximum real frequency used to calculate the spectral function
from the GW Green's function. freqspmax and nfreqsp
define the spacing of the frequency mesh along the real axis.
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gwcalctyp
Mnemonics: GW CALCulation TYPe
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
gwcalctyp governs the choice between the different capabilities of the GW code.
- 0 <= gwcalctyp <= 9 : standard "1 shot" quasiparticle method
- 10 <= gwcalctyp <= 19 : self-consistent quasiparticle method on energies only
- 20 <= gwcalctyp <= 29 : self-consistent quasiparticle method on energies and wavefunctions
- gwcalctyp = 0, 10, or 20 : standard Plasmon-Pole model GW calculation
- gwcalctyp = 2, 12, or 22 : GW calculation using numerical integration (contour deformation method, see e.g. S. Lebegue et al. PRB 67, 155208 (2003).)
- gwcalctyp = 5, 15, or 25 : Hartree-Fock calculation
- gwcalctyp = 6, 16, or 26 : Screened Exchange calculation
- gwcalctyp = 7, 17, or 27 : COHSEX calculation
- gwcalctyp = 8, 18, or 28 : model GW calculation following S. Faleev et al. PRL 93, 126406 (2004) using a Plasmon-Pole model
- gwcalctyp = 9, 19, or 29 : model GW calculation following S. Faleev et al. PRL 93, 126406 (2004) using numerical integration (contour deformation method)
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gwcomp
Mnemonics: GW COMPletness
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
gwcomp governs the use of an extrapolar approximation. If gwcomp==1, one improves the completeness
in a truncated sum over states. In practice, this permits one to reduce quite much the number of
bands required in the calculation of the screening or of the self-energy.
The energy parameter
needed in the extrapolar approximation is set by gwencomp.
See F. Bruneval, X. Gonze, Phys. Rev. B 78, 085125 (2008) for a description of the methodology.
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gwencomp
Mnemonics: GW Energy for COMPletness
Characteristic: GW
Variable type: energy
Default is 2.
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations, when gwcomp is equal to 1.
gwencomp sets the energy parameter used in the extrapolar approximation used to improve completeness and make the convergence against the number of bands much faster.
See F. Bruneval, X. Gonze, Phys. Rev. B 78, 085125 (2008) for a description of the methodology.
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gwgamma
Mnemonics: GW Gamma
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=4, that is, sigma calculations.
If gwgamma is 1, the vertex correction will be included leading to what is known as "GWGamma" approximation. see R. Del Sole, L. Reining, and R. W. Godby, Phys. Rev. B 49, 8024 (1994). Note that, in order to include the vertex correction in W, one has to start the sigma calculation from the susceptibility file_SUSC instead of the _SCR file (see getsuscep and irdsuscep ) Not available for PAW calculations.
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gwmem
Mnemonics: GW MEMory
Characteristic: GW
Variable type: integer
Default is 11
Only relevant if optdriver=3,4, that is, sigma calculations.
gwmem governs the memory strategy during a screening and/or a sigma run.
- gwmem = 1x , the screening matrix are read for all q-vectors and stored in the memory.
- gwmem = 0x , the screening matrix are read just a q-vector after another.
- gwmem = x1 , the real-space wavefunctions are stored in the memory.
- gwmem = x0 , the real-space wavefunctions are not stored, but rather recalculated on-fly each abinit needs them using FFTs.
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gwrpacorr
Mnemonics: GW RPA CORRelation energy
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=3 and if gwcalctyp=x1, i.e. a screening calculation for imaginary frequencies.
gwrpacorr governs the calculation of the RPA correlation energy.
- gwrpacorr = 0, no RPA correlation energy is calculated
- gwrpacorr = 1, the RPA correlation energy is calculated using an exact integration over the coupling constant: it requires one diagonalization of the polarizability matrix
- gwrpacorr = n > 1, the RPA correlation energy is calculated using n values for the coupling constant: it requires n inversions of the polarizability matrix
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gw_eet
Mnemonics: GW Effective Energy Technique
Characteristic: GW
Variable type: integer
Default is -1
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
gw_eet governs the use of the effective energy technique (EET). With the EET GW calculations can be performed without summing over the empty states, both in the calculation of the screening and the self-energy. This is achieved by using a single effective energy that takes into account the contributions of all the empty states. See J.A. Berger, L. Reining, and, F. Sottile, Phys. Rev. B (82), 041103(R) (2010) for a more detailed description of the method.
- gw_eet = -1, no effective energy technique is used
- gw_eet = 0, 1, or 2 an effective energy is used. These numbers correspond to the order of the approxiation for the effective energy as given in Eqs. (10), (11), and (12), respectively, in J.A. Berger, L. Reining, and, F. Sottile, Phys. Rev. B (82), 041103(R) (2010). The higher the order, the more accurate (in principle) is the result. However, also the calculation time of the effective energy will increase.
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gw_eet_inclvkb
Mnemonics: GW Effective Energy Technique INCLude VKB
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations and if gw_eet=1 or 2
gw_eet_inclvkb governs the use of the commutator of the non-local part of the pseudo-potential in determining the effective energy of the EET.
- gw_eet_inclvkb = 0 the commutator is not included
- gw_eet_inclvkb = 1 the commutator is included
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gw_eet_nband
Mnemonics: GW Effective Energy Technique Number of BANDs
Characteristic: GW
Variable type: integer
Default is -1
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations and if gw_eet=0, 1, or 2
gw_eet_nband governs the number of bands to be used in the effective energy technique (EET). It will only be taken into account if it is higher than the number of occupied bands. If this is not the case, summations will only include occupied states.
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gw_eet_scale
Mnemonics: GW Effective Energy Technique SCALE
Characteristic: GW
Variable type: integer
Default is -1
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations and if gw_eet=0, 1, or 2
gw_eet_scale sets the prefactor with which the effective energy can be scaled to
obtain a faster convergence with respect to the number of empty states.
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gw_nstep
Mnemonics: GW Number of self-consistent STEPS
Characteristic: GW
Variable type: integer
Default is 30
Only relevant if optdriver=8, that is, self-consistent GW calculations.
Gives the maximum number of self-consistent GW cycles (or "iterations").
in which G and/or W will be updated until the quasi-particle energied are converged
within gw_toldfeig.
gwcalctyp and gw_sctype are
used to define the type of self-consistency.
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gw_nqlwl
Mnemonics: GW, Number of Q-points for the Long Wave-Length Limit
Characteristic: GW
Variable type: integer
Default is 1
Only relevant if optdriver=3,4,99 that is, screening, sigma or BS calculations, althought the actual meaning of the variable depends on the particular run-level (see discussion below).
gw_nqlwl defines the number of directions in reciprocal space used to describe the non-analytical behaviour of the heads (G = G'=0) and the wings (G=0 or G'=0) of the dielectric matrix in the optical limit (i.e. for q tending to zero). The number of directions is specified by the additional variable gw_qlwl.
When optdriver=3, gw_nqlwl and gw_qlwl define the set of "small" q that will be calculated and stored in the final SCR file. Therefore, the two variables can be used to analyze how the optical spectra depend on the direction of the incident phonon (useful especially in anisotropic systems).
When optdriver=4, gw_nqlwl and gw_qlwl can be used to specify the heads and the wings to be used to perform the quadrature of the correlated part of the self-energy in the small region around the origin. (NB: not yet available, at present the quadrature is performed using a single direction in q-space)
When optdriver=99, gw_nqlwl and gw_qlwl define
the set of directions in q-space along which the macroscopic dielectric function is evaluated.
By default the BS code calculates the macroscopic dielectric function using six different
directions in q-space (the three basis vectors of the reciprocal lattice and the three Cartesian axis).
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gw_qlwl
Mnemonics: GW, Q-points for the Long Wave-Length Limit
Characteristic: GW
Variable type: real array gw_qlwl(3,gw_nqlwl)
Default is 0.00001 0.00002 0.00003
Only relevant if optdriver=3,4 that is, screening or sigma calculations.
When optdriver=3, gw_qlwl defines the set of q-points around Gamma that are considered during the evaluation of the non-analytical behaviour of the dielectrix matrix. Optical spectra (with and without non-local field effects) are evaluated for each direction specified by gw_qlwl.
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gw_sctype
Mnemonics: GW, Self-Consistency TYPE
Characteristic: GW
Variable type: integer
Default is 0 so that no choice is done
This variable is used to partially define the kind of self-consistency for GW calculations. The other piece of information is given by gwcalctyp that defines the particular approximation for the self-energy operator as well as whether the wavefunctions have to replaced by quasi-particle amplitudes.
If gw_sctype is specified in the input file, the code will perform an iterative update of the quantities entering the GW equations until the quasi-particle energies are converged within gw_toldfeig. The maximum number of iterations is specified by gw_nstep. Possible values are:
- 1 => standard one-shot method (one screening calculation followed by a single sigma run)
- 2 => self-consistency only on W (iterative update of W followed by a sigma run in which G is approximated with the Kohn-Sham independent-particle Green's function G0)
- 3 => self-consistency only of G (a single screening calculation to obtain the Kohn-Sham polarizability followed by an iterative update of the Green's functions in the self-energy)
- 4 => fully self-consistent algorithm (iterative update of both G and W)
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gw_sigxcore
Mnemonics: GW, treatment of the ...
Characteristic: GW
Variable type: integer
Default is 0
Only available for PAW and relevant if optdriver=4 that is, sigma calculations.
Theoretical introduction: GW calculations perfomed on top of electronic calculations relying when the frozen-core approximation is used to separate inner-core electrons from valence electrons, only the contribution to the self-energy arising from valence electrons is explicitly accounted for. In the standard approach based on pseudopotentials the contribution to the self-energy due to core electrons is approximated by means of the KS exchange-correlation potential generated by the core density. In the case of GW calculations employing the PAW method, the core contribution to the self-energy can be more accurately estimated in terms of the Fock operator generated by the core wavefunctions. In the simplest approach, the only ingredients required for this more refined treatment are the wave functions of the core electrons in the reference atomic configuration that are calculated during the generation of the PAW setup. This is a good approximation provided that the core wave functions are strictly localized inside the PAW spheres.
gw_sigxcore defines the approximation used to evaluate the core contribution to sigma.
- gw_sigxcore = 0, standard approach, the core contribution is approximated with vxc.
- gw_sigxcore = 1, the core term is approximated with the Fock operator inside the PAW spheres.
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gw_toldfeig
Mnemonics: GW TOLerance on the DiFference of the EIGenvalues
Characteristic: GW, ENERGY
Variable type: REAL
Default is 0.1 eV
Sets a tolerance for absolute differences of QP energies that will cause one self-consistent GW cycle to stop.
Can be specified in Ha (the default), Ry, eV or Kelvin, since
toldfe has the 'ENERGY' characteristics (1 Ha=27.2113845 eV)
Effective only when self-consistent GW calculations are done (optdriver=8).
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inclvkb
Mnemonics: INCLude VKB
Characteristic: GW
Variable type: integer parameter
Default is 1 TODO: should be changed to 0 or 2
Only relevant if optdriver=3,99 that is, a screening or BS calculations.
Possible values of inclvkb are 0,1,2. If inclvkb is 1 or 2, the commutator of the non-local part of the pseudopotential with the position operator is correctly included in the q => 0 contribution. This is unfortunately time-consuming. When inclvkb is 0, this contribution is incorrectly omitted, but the computation is much faster.
The importance of this contribution depends on the number of k points. Turning off inclvkb should be made by experienced users only.
The use of inclvkb=2 is strongy recommended for the calculation of optical properties.
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icutcoul
Mnemonics: Integer that governs the CUT-off for COULomb interaction
Characteristic: GW
Variable type: integer parameter
Default is 3
(This documentation is still very primitive, and should be checked) Only relevant if optdriver=3 or 4, that is, a screening or self-energy calculation.
Many-body calculations for isolated systems present a slow convergence with respect to the size of the supercell due to the long ranged Coulomb interaction and the high degree of non-locality of the operators involved. A similar issue occur also in fully periodic systems due to the presence of the integrable Coulomb singularity at G=0 that hinders the convergence with respect to the number of q-points used to sample the Brillouin zone. The convergence can be accelerated by replacing the true bare Coulomb interaction with other expressions icutcoul defines the particular expression to be used for the Coulomb term in reciprocal space. The choice of icutcoul depends on the dimensionality of the system. Possible values of icutcoul are 0 to 6. To be complemented by values of vcutgeo and rcut
- 0 => sphere (molecules but also 3D-crystals)
- 1 => cylinder (nanowires, nanotubes)
- 2 => surface
- 3 => 3D crystal (no cut-off, G=0 excluded, default value)
- 4 => ERF, long-range only Coulomb interaction
- 5 => ERFC, short-range only Coulomb interaction (e.g. as used in the HSE functional)
- 6 => auxiliary function integration for 3D systems from P. Carrier et al., PRB 75, 205126 (2007).
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kptgw
Mnemonics: K-PoinTs for GW calculations
Characteristic: GW
Variable type: real array kptgw(3,nkptgw)
Default is all 0.0's
Only relevant if optdriver=4, that is, sigma calculations.
For each k-point with number igwpt in the range (1:nkptgw), kptgw(1,igwpt) is the reduced coordinate of the k-point where GW corrections are required. while bdgw(1:2,igwpt) specifies the range of bands to be considered.
At present, not all k-points are possible. Only those corresponding to the k-point
grid defined with the same repetition parameters (kptrlatt,
or ngkpt) than the GS one, but WITHOUT any shift, are allowed.
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nbandkss
Mnemonics: Number of BaNDs in the KSS file
Characteristic:
Variable type: integer parameter
Default is 0
This input variable is used for the preparation of a GW calculation : it is used in a GS run (where optdriver=0) to generate a _KSS file. In this run, nbandkss should be non-zero. The generated _KSS file can be subsequently used to calculate the irreducible polarizabilty $\chi^{(0)}_{KS}$ using optdriver=3 or to calculate GW corrections setting optdriver=4.
- If nbandkss=0, no _KSS file is created
- If nbandkss=-1, all the available eigenstates (energies and eigenfunctions) are stored in the abo_KSS file at the end of the ground state calculation. The number of states is forced to be the same for all k-points : it will be the minimum of the number of plane waves over all k-points.
- If nbandkss is greater than 0, abinit stores (about) nbandkss eigenstates in the abo_KSS file. This number of states is forced to be the same for all k-points.
See npwkss for the selection of the number of the planewave components of
the eigenstates to be stored.
The input variable accesswff can be used
to read and write KSS files according to different fileformat
(presently only accesswff=0 and 3 are available in the GW part).
The precision of the KSS file can be tuned through the input variable kssform.
For more details about the format of the abo_KSS file, see the routine outkss.F90.
Very important : for the time being, istwfk must be 1 for all the k-points
in order to generate a _KSS file.
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nfreqim
Mnemonics: Number of FREQuencies along the IMaginary axis
Characteristic:
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3, that is, screening calculations.
nfreqim is used only for numerical integration of the GW self-energy
(gwcalctyp= 2, 12, 22, 9, 19, 29).
nfreqim sets the number of pure imaginary frequencies used to calculate
the dielectric matrix in order to perform the numerical integration of the GW self-energy.
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nfreqmidm
Mnemonics: Nth FREQuencey moment of the IMaginary part of DM
Characteristic:
Variable type: integer parameter
No default
at the moment only relevant if optdriver=4, that is, a screening calculation.
depending on the value of >nfreqmidm will calculate the frequency moment of the Dielectric matrix or its inverse,
if >nfreqmidm positive : calculate (nth=nfreqmidm) frequency moment of the Dielectric matrix
if >nfreqmidm negative : calculate (nth=nfreqmidm) frequency moment of the inverse Dielectric matrix
if >nfreqmidm = 0 : calculate first frequency moment of the full polarizability
see M. Taut, J. Phys. C: Solid State Phys. 18 (1985) 2677-2690.
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nfreqre
Mnemonics: Number of FREQuencies along the Real axis
Characteristic:
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3, that is, screening calculations.
nfreqre is used only for numerical integration of the GW self-energy
(gwcalctyp= 2, 12, 22, 9, 19, 29).
nfreqre sets the number of real frequencies used to calculate
the dielectric matrix in order to perform the numerical integration of the GW self-energy.
It can be used also in case of GW calculations with plasmon-pole models,
i.e gwcalctyp<10,
to reduce the number of frequencies used to evaluate the dielectric matrix from the (default) two to
one frequency (omega=0) by setting nfreqre=1.
This might be a good idea in case one is planning to use ppmodel>1.
This will force the calculation of the screening on a single frequency (omega=0) and hence
reduce memory and disk space requirement.
The only draw back is that the user will not be able to perform self energy calculation using
ppmodel=1,
since in the last case the dielectric matrix calculated on two frequencies is required.
If the user is not sure which ppmodel to use, then s/he is not advised
to use this input variable.
Using the default values, one must be able to get a screening file that can be used with any ppmodel.
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nfreqsp
Mnemonics: Number of FREQuencies for the SPectral function
Characteristic:
Variable type: integer parameter
Default is 0
Only relevant if optdriver=4, that is, sigma calculations.
nfreqsp defines the number of real frequencies used to calculate the spectral function
of the GW Green's function.
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npwkss
Mnemonics: Number of PlaneWaves in the KSS file
Characteristic:
Variable type: integer parameter
Default is 0
This input variable is used for the preparation of a GW calculation: the GS run (where optdriver=1 and nbandkss/=0) should be followed with a run where optdriver=3. Also, if nbandkss=0, no use of npwkss.
npwkss defines the number of planewave components of the Kohn-Sham states to build the Hamiltonian, in the routine outkss.F90, and so, the size of the matrix, the size of eigenvectors, and the number of available states, to be stored in the abo_KSS file. If it is set to 0, then, the planewave basis set defined by the usual Ground State input variable ecut is used to generate the superset of all planewaves used for all k-points. Note that this (large) planewave basis is the same for all k-points.
Very important : for the time being, istwfk must be 1 for all the k-points.
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nkptgw
Mnemonics: Number of K-Points for GW corrections
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=4, that is, sigma calculations. This input variable was called "ngwpt" in versions before v4.3.
nkptgw gives the number of k-points for which the GW calculation must be done.
It is used to dimension kptgw
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nomegasi
Mnemonics: Number of OMEGA(S) along the Imaginary axis
Characteristic: GW
Variable type: integer
Default is 12
Only relevant for sigma calculations in which the self-energy along the real axis is obtained by performing the analytic continuation from the imaginary axis to the full complex plane via the Pade approximant (optdriver=4 and gwcalctyp=x1).
nomegasi defines the number of frequency points used to sample the self-energy along the
imaginary axis. The frequency mesh is linear and covers the interval between OMEGASIMIN=0.01 Hartree and
omegasimax.
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nomegasf
Mnemonics: Number of OMEGA to evaluate the Spectral Function
Characteristic: GW, ENERGY
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3 and spmeth/=0, that is, screening calculations based on the spectral reprentation of the irreducible polarizability.
nomegasf defines the number of real frequencies used to describe the spectral function associated to the irreducible polarizability $\chi^{(0)}_{KS}$. The frequency mesh will cover the interval between 0 and the maximum (positive) transition energy between occupied and empty states. The delta function entering the expression defining the spectral function is approximated using two different methods according to the value of the spmeth input variable.
It is important to notice that an accurate description of the imaginary part of $\chi^{(0)}_{KS}$
requires an extremely dense frequency mesh. It should be kept in mind, however, that the memory required
grows fast with the value of nomegasf.
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nomegasrd
Mnemonics: Number of OMEGA to evaluate the Sigma Real axis Derivative
Characteristic: GW
Variable type: integer parameter
Default is 9
Only relevant if optdriver=4, that is, sigma calculations.
The number of real frequencies around the KS energy where the self-energy Sigma is evaluated.
From these values, the derivative of Sigma at the KS energy is numerically estimated through linear interpolation.
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npweps
Mnemonics: Number of PlaneWaves for EPSilon (the dielectric matrix)
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
npweps determines the size of the planewave set used to represent the independent-particle
susceptibility $\chi^{(0)}_{KS}$, the dielectric matrix $\epsilon$ and its inverse.
See ecuteps (preferred over npweps) for more information.
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npwsigx
Mnemonics: Number of PlaneWaves for SIGma eXchange
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=4, that is, sigma calculations.
npwsigx determines the cut-off energy of the planewave set used to generate the
exchange part of the self-energy operator.
See ecutsigx (preferred over npwsigx) for more information.
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npwwfn
Mnemonics: Number of PlaneWaves for WaveFunctioNs
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3 or 4, that is, screeening or sigma calculations.
npwwfn determines the size of the planewave set used to represent the wavefunctions
in the formula that generates the independent-particle susceptibility $\chi^{(0)}_{KS}$.
See ecutwfn (preferred over nshwfn) for more information.
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nqptdm
Mnemonics: Number of Q-PoinTs for the Dielectric Matrix
Characteristic:
Variable type: integer parameter
Default is 0
Used only in the screening part, that is for optdriver=3.
If nqptdm is equal to 0, the set of q-points for computing the dielectric matrix is
determined automatically considering all the possible differences between the k-points contained
in the _KSS file. When nqptdm is non-zero, the list of q points is read from qptdm.
This allows one to split the big calculation of all the dielectric matrices into smaller
calculations that can be performed independently. The _SCR files generated in different runs
can be merged thanks to the Mrgscr utility.
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nsheps
Mnemonics: Number of SHells for EPSilon (the dielectric matrix)
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3, that is, screening calculations.
nsheps determines the size of the planewave set used to represent the independent-particle
susceptibility $\chi^{(0)}_{KS}$, the dielectric matrix $\epsilon$ and its inverse.
See ecuteps (preferred over nsheps) for more information.
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nshsigx
Mnemonics: Number of SHells for SIGma eXchange
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=4, that is, sigma calculations.
nshsigx determines the cut-off energy of the planewave set used to generate the
exchange part of the self-energy operator.
See ecutsigx (preferred over nshsigx) for more information.
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nshwfn
Mnemonics: Number of SHells for WaveFunctioNs
Characteristic: GW
Variable type: integer parameter
Default is 0
Only relevant if optdriver=3 or 4, that is, screening or sigma calculations.
nshwfn determines the number of shells of the planewave set used to represent the wavefunctions
in the formula that generates the independent-particle susceptibility $\chi^{(0)}_{KS}$.
See ecutwfn (preferred over nshwfn) for more information.
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omegasimax
Mnemonics: OMEGA to evaluate Sigma along the Imaginary axis D: MAXimal value
Characteristic: GW, ENERGY
Variable type: real
Default is 50 eV
Only relevant for sigma calculations in which the self-energy along the real axis is obtained by performing the analytic continuation from the imaginary axis to the full complex plane via the Pade approximant (optdriver=4 and gwcalctyp=x1).
omegasimax defines the maximum frequency along the imaginary the axis.
In conjunction with nomegasi,
omegasimax uniquely defines the linear mesh employed to sample the self-energy along the imaginary axis.
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omegasrdmax
Mnemonics: OMEGA to evaluate the Sigma Real axis Derivative : MAXimal value
Characteristic: GW, ENERGY
Variable type: real
Default is 1.0 eV
Only relevant if optdriver=4, that is, sigma calculations.
The maximum distance from the KS energy where to evaluate Sigma. Sigma is evaluated at
[KS_energy - maxomegasrd, KS_energy + maxomegasrd] sampled nomegasrd times.
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ppmfrq
Mnemonics: Plasmon Pole Model FReQuency
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0 Ha
Only relevant if optdriver=3 or 4, that is, screening calculations or sigma calculations. Only effective if GW corrections are evaluated using the plasmon-pole model of Godby-Needs (i.e ppmodel=1)
In the present status of the GW code, the convolution in frequency space
defining the self-energy operator can be evaluated using two different approaches:
numerical integration and plasmon-pole models.
Methods based on the numerical integration (contour deformation, analytic continuation) require
the knowledge of the screened interaction for several frequencies. These approaches give
the most accurate results but at the price of an increase in the CPU time required.
Alternately it is possible to approximate the dynamical behaviour of the screened interaction
through simple analytical expressions, the so-called plasmon-pole models.
In the plasmon-pole model proposes by Godby-Needs (ppmodel=1),
the screening must be available at zero frequency, as well as at another frequency, imaginary,
of the order of the plasmon frequency (the peak in the EELS spectrum).
This information is used to derive the behaviour of the dielectric matrix
for all the frequencies (complex).
During the calculation of the screening, ppmfrq defines the imaginary frequency where the
dielectric matrix is evaluated, in addition to the zero frequency.
During the self-energy run, ppmfrq can be used to define the second frequency to be used
to calculate the plasmon-pole parameters. This is particularly useful when the
SCR file contains several frequencies along the imaginary axis.
In this case the frequency whose value is the closest one to ppmfrq will be selected.
Note that, if the plasmon-pole approximation is good, then, the
choice of ppmfrq should have no influence on the final result.
One should check whether this is the case. In general, the plasmon frequencies of bulk solids
are of the order of 0.5 Hartree.
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ppmodel
Mnemonics: Plasmon Pole MODEL
Characteristic: GW
Variable type: integer parameter
Default is 1
Only relevant if optdriver=3 or 4, that is, screening calculations and self-energy calculations.
- ppmodel=1 : PP model of Godby and Needs, See FIXME>REFERENCE MISSING!
- ppmodel=2 : PP model of Hybertsen and Louie, See Phys Rev B 34, 5390 (1986)
- ppmodel=3 : PP model of W. von der Linden and P. Horsh see Phys Rev B 37, 8351 (1988)
- ppmodel=4 : PP model of Farid and Engel. See Phys Rev B47,15931 (1993)
- ppmodel=0 : no PP model, numerical integration (contour deformation method, see e.g. S. Lebegue et al. PRB 67, 155208 (2003).)
Please note also that in the case of ppmodel 4, the plasmon energies are not simple mathematical parameters,
but rather have a physical meaning (at least the lowest ones).
Thus the calculated plasmon band structure (plasmon energy vs q vector) is reported in the output file for the
lowest 10 bands.
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qptdm
Mnemonics: Q-PoinTs for the Dielectric Matrix
Characteristic: GW
Variable type: real array qptdm(3,nqptdm)
Default is 0. 0. 0. (for just one q-point)
Only relevant if optdriver=3, that is, screening calculations, and only if nqptdm/=0.
qptdm contains the set of q-points used in the screening part of ABINIT,
instead of the automatic generation of the q points when nqptdm=0.
These q points are given in terms of reciprocal space primitive translations (NOT in cartesian coordinates!).
For further explanation, see the input variable nqptdm.
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rcut
Mnemonics: Radius of the CUT-off for coulomb interaction
Characteristic: GW
Variable type: real parameter
Default is 0.0d0
Truncation of the Coulomb interaction in real space. The meaning of rcut is governed by the cutoff shape option icutcoul.
If rcut is negative, the cutoff is automatically calculated so to enclose the same volume inside the cutoff as the volume of the solid.
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rhoqpmix
Mnemonics: RHO QuasiParticle MIXing
Characteristic: GW
Variable type: real parameter
Default is 1.0
For self-consistent GW runs, rhoqpmix sets the mixing coefficient between the new and the previous
electronic densities.
This mixing damps the spurious oscillations in the Hartree potential when achieving self-consistency.
rhoqpmix is meaningful only when doing self-consistency on the wavefunctions with
gwcalctyp >= 20.
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soenergy
Mnemonics: Scissor Operator ENERGY
Characteristic: GW, ENERGY
Variable type: real
Default is 0.0
Only relevant if optdriver=3,4,99 that is, screening, sigma or Bethe-Salpeter calculations.
The Scissors operator energy added to the conductions states.
In some cases, it mimics a second iteration self-consistent GW calculation.
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spbroad
Mnemonics: SPectral BROADening
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.0
Only relevant if optdriver=3 and spmeth=2, that is, screening calculations based on the spectral reprentation of the irreducible polarizability in which the delta function is replaced by the gaussian approximant whose standard deviation is given by spbroad.
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spmeth
Mnemonics: SPectral METHod
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=4, that is, sigma calculations.
The spmeth input variable defines the method used to calculate the irreducible polarizability $\chi^{(0)}_{KS}$.
By default $\chi^{(0)}_{KS}$ is calculated employing the Adler-Wiser expression (spmeth=0)
with a CPU effort that scales linearly with the number of frequencies.
This approach is convenient when few frequencies are required, and is usually
used in conjunction with plasmon-pole models in which only one or two frequencies are calculated,
according to the value of ppmodel.
Unfortunately a calculation based on the Adler-Wiser expression might be quite CPU demanding
if the matrix elements of the self-energy operator are evaluated by performing numerically
the convolution defining the self-energy.
The integrand function, indeed, has poles above and below the real axis, and
the screened interaction has to be evaluated on a dense frequency mesh in order to obtain accurate
results.
In the spectral method (spmeth=1 or 2) the irreducible polarizability is expressed as the Hilbert transform of the imaginary part. The advantage in using this approach consists in the fact that, once the spectral function is known, the irreducible polarizability for an arbitrary frequency can be easily obtained through inexpensive integrations. On the other hand an accurate evaluation of the imaginary part requires a dense frequency mesh due to the presence of delta functions. Two different approaches can be used to approximate these delta functions thus allowing the use of affordable frequency grids.
Summarizing:
- 0 => use Adler-Wiser expression to calculate $\chi^{(0)}_{KS}$
- 1 => use the spectral method approximating the delta function with a triangular approximant as proposed in REF TO BE ADDED
- 2 => use spectral method but approximating the delta function with a Taylor expansion of the exponential as proposed in REF TO BE ADDED
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symchi
Mnemonics: SYMmetryze \chi_o
Characteristic: DEVELOP, GW
Variable type: integer
Default is 1
Only relevant if optdriver=3, that is, screening calculations.
The evaluation of the irreducible polarizability for a given q-point requires an integration over the Brillouin zone (BZ) which is approximated by a discrete sum over k-points. In principle the integrand function should be evaluated for each k-point in the BZ, however it is possible to reduce the number of points to be explicitly considered by taking advantage of symmetry properties. The development input variable symchi is used to choose between these two equivalent methods:
- 0=> the summation over k-points is performed considering ALL the points in the BZ (useful for testing and debugging).
- 1=> the summation is restricted to the k-points belonging to the irreducible wedge defined by the little group associated to the external vector q.
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symmorphi
Mnemonics: SYMMORPHIc symmetry operations
Characteristic: DEVELOP, GW
Variable type: integer parameter
Default is 1
With symmorphi=1, symmetry operations with a non-symmorphic vector are allowed. With symmorphi=0, they are not allowed. In the latter case, if the symmetry operations are specified in the input file, the code will stop and print an error message if a non-symmorphic vector is encountered. By contrast, if the symmetry operations are to be determined automatically (if nsym=0), then the set of symmetries will not include the non-symmorphic operations.
Note : this feature exist because in a previous status of the GW calculations, non-symmorphic
symmetry operations could not be exploited. Thus, the k points were restricted
to the IBZ. In order to prepare GW calculations, and to perform GW calculations,
symmorphi=0 was to be used, together with nsym=0.
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symsigma
Mnemonics: SYMmetrization of SIGMA matrix elements
Characteristic: GW
Variable type: integer
Default is 0
Only relevant if optdriver=4 that is sigma calculations.
This option is used to switch on the symmetrization of the self-energy matrix elements (symsigma=1). In this case the BZ integration defining the self-energy matrix elements is reduced to an appropriate irreducible wedge defined by the point group of the wave-vector k specified in the kptgw list.
The symmetrized expression leads to a considerable speedup of the run but, unfortunately, this option is not yet compatible with self-consistent GW calculations (see gwcalctyp).
The algorithm implemented in symsigma=1
constructs a symmetric invariant for the diagonal matrix elements of the self-energy
by simply averaging the GW results within the degenerate subspace.
Therefore particular care has to be taken in the presence of accidental degeneracies.
since GW calculations performed with symsigma=1 won't be able to remove
the initial accidental degeneracy.
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vcutgeo
Mnemonics: V (potential) CUT-off GEOmetry
Characteristic: GW
Variable type: real array vcutgeo(3)
Default is 3*0.0d0
(No explicit documentation, please see https://listes-2.sipr.ucl.ac.be/abinit.org/arc/forum/2008-11/msg00087.html for the time being)
vcutgeo is used in conjunction with icutcoul to specify the geometry used to truncate the Coulomb interaction as well as the particular approach to be used. It has a meaning only for the cylindrical symmetry (icutcoul=1) or in the case of surfaces icutcoul=2. For each geometry, two different definition of the cutoff region are available (see Phys. Rev. B 73, 233103 and Phys. Rev. B 73, 205119 for a complete description of the methods)
In Beigi's method (Phys. Rev. B 73, 233103), the cutoff region is given by the Wigner-Seitz cell centered on the axis of the cylinder. The cutoff region is thus automatically defined by the unit cell and there's no need to specify When rcut.
To define a cylinder along the z-axis use the following two lines. icutcoul 1 rprim 1 0 0 0 1 0 0 0 1 vcutgeo 0 0 1
Please note that Beigi's method is implemented only in the case if an orthorombic Bravais lattic. For hexagonal lattices, one has to use the method of Rozzi (Phys. Rev. B 73, 205119) In this case, the interaction is truncated in a finite cylinder. Contrarily to the first approach, here one has to specify both the radius of the cylinder with rcut as well as the length of the cylinder along the periodic dimension that should always be smaller than the extension of the Born von Karman box. The length of the cylinder is given in terms of the primitive vector along the periodic direction.
For example, in order to define a finite cylinder along z of radius 2.5 Bohr and length 3*R3 icutcoul 1 rprim 1 0 0 0 1 0 0 0 1 vcutgeo 0 0 -3.0 # note the minus sign rcut 2.5
For surface calculations (icutcoul=2),
vcutgeo is used to define the two periodic directions defining the surface.
Also in this case two different techniques are available.
In the method of Beigi, the (positive) non-zero components of vcutgeo define the periodic
directions of the infinite surface. The interaction is truncated within a slab
of width L where L is the length of the primitive vector of the lattice along the non-periodic dimension.
For example:
icutcoul 2
rprim 1 0 0
0 1 0
0 0 1
vcutgeo 1 1 0
It is also possible to define a finite surface by employing negative values
For example:
icutcoul 2
vcutgeo -3 -2 0
defines ....
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zcut
Mnemonics: Z-CUT
Characteristic: GW, ENERGY
Variable type: real parameter
Default is 0.1 eV = 3.67493260d-03Ha
Only relevant if optdriver=3,4,99 that is, screening, sigma or BS calculations.
It is meant to avoid some divergencies that might occur during the evaluation of the Adler-Wiser expression of the irreducible polarizability (optdriver=3) or during the numerical treatment of the integrals defining the contribution to the self-energy matrix elements (optdriver=4). If the denominator becomes smaller than zcut, a small imaginary part (depending on zcut) is added, in order to avoid the divergence.
When optdriver=99, zcut defines the small complex shift
used to avoid divergences in the expression for the macroscopic dieletric function.
It simulates the experimental uncertainty and the finite lifetime of the quasiparticles
(although the true lifetime should be k- and band-dependent).
The value of zcut affects the number of iteration needed to achieve convergence
in the Haydock iterative method. In this case, zcut should be
larger than the typical distance between the eigenvalues of the exciton Hamiltonian.
Ideally, one should make a convergence study decreasing the value of zcut for increasing number of k-points.
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ABINITv6.12.3 Production release
