DFT performs reasonably well for the determination of structural properties, but fails to predict accurate band gaps. A more rigorous framework for the description of excited states is provided by many-body perturbation theory (MBPT) [Fetter1971],[Abrikosov1975], based on the Green's functions formalism and the concept of quasi-particles [Onida2002].
Within MBPT, one can calculate the quasi-particle (QP) energies, E, and amplitudes, Ψ, by solving a nonlinear equation involving the non-Hermitian, nonlocal and frequency dependent self-energy operator Σ.
This equation goes beyond the mean-field approximation of independent KS particles as it accounts for the dynamic many-body effects in the electron-electron interaction.
Details about the GW implementation in ABINIT can be found here.
A typical GW calculation consists of two different steps (following a DFT calculation): first the screened interaction ε-1 is calculated and stored on disk (optdriver=3), then the KS band structure and W are used to evaluate the matrix elements of Σ, finally obtaining the QP corrections (optdriver=4).
The computation of the screened interaction is described in topic_Susceptibility, while the
computation of the self-energy is described in topic_SelfEnergy. The frequency meshes, used e.g. for
integration along the real and imaginary axes are described in topic_FrequencyMeshMBPT.
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Compulsory input variables:
... optdriver [OPTions for the DRIVER]
Basic input variables:
... bdgw [BanDs for GW calculation]
... gw_nstep [GW Number of self-consistent STEPs]
... gw_sctype [GW, Self-Consistency TYPE]
... gw_toldfeig [GW TOLerance on the DiFference of the EIGenvalues]
... gwcalctyp [GW CALCulation TYPe]
... gwpara [GW PARAllelization level]
Useful input variables:
... getqps [GET QuasiParticle Structure]
... getscr [GET SCReening (the inverse dielectric matrix) from ...]
... getsuscep [GET SUSCEPtibility (the irreducible polarizability) from ...]
... gwfockmix [GW FOCK exchange MIXing parameter]
... irdqps [Integer that governs the ReaDing of QuasiParticle Structure]
... irdscr [Integer that governs the ReaDing of the SCReening]
... irdsuscep [Integer that governs the ReaDing of the SUSCEPtibility]
... mbpt_sciss [Many Body Perturbation Theory SCISSor operator]
... nbandkss [Number of BANDs in the KSS file]
... nsym [Number of SYMmetry operations]
... rhoqpmix [RHO QuasiParticle MIXing]
... symmorphi [SYMMORPHIc symmetry operation selection]
... usepawu [USE PAW+U (spherical part)]
Input variables for experts:
... fftgw [FFT for GW calculation]
... gw_nqlwl [GW, Number of Q-points for the Long Wave-Length Limit]
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tests/v5/Input: t65.in
t66.in
t69.in
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