Density-Functional Perturbation Theory (DFPT) allows one to address a large variety of physical observables. Many properties of interest can be computed directly from the derivatives of the energy, without the use of finite differences: phonons modes, elastic tensors, effective charges, dielectric tensors, etc... Even non-linear properties can be computed, like the Raman intensities (for the latter, see topic_nonlinear)..
A DFPT calculation workflow is conducted as follows:
Note that for PAW calculation, when performing the post-processing with anaddb, it is recommended to include all the keywords enforcing the sum rules (acoustic sum and charge neutrality). Indeed the PAW formalism involves, for each atom, the calculation of a large number of real space integrals, whose numerical effect may be to break the translational invariance.
Thanks to the locality provided by PAW partial wave basis, it is possible to perform response function calculations for correlated electron materials. The LDA+U formalism is usable without any restriction for the PAW+DFPT calculations.
All the tutorials dedicated to response functions can be followed both with norm-conserving pseudopotentials and with PAW atomic datasets.
More detailed explanations to perform a response calculation are given in the help_respfn file.
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Compulsory input variables:
... optdriver [OPTions for the DRIVER]
... rfdir [Response Function : DIRections]
Basic input variables:
... rfatpol [Response Function : ATomic POLarisation]
... rfddk [Response Function with respect to Derivative with respect to K]
... rfelfd [Response Function with respect to the ELectric FielD]
... rfmagn [Response Function with respect to MAGNetic B-field perturbation]
... rfphon [Response Function with respect to PHONons]
... rfstrs [Response Function with respect to STRainS]
Useful input variables:
... asr [Acoustic Sum Rule]
... dfpt_sciss [DFPT SCISSor operator]
... getwfkfine [GET the fine grid wavefunctions from _WFK file]
... ird1wf [Integer that governs the ReaDing of _1WF files ]
... irdddk [Integer that governs the ReaDing of DDK wavefunctions, in _1WF files]
... irdwfq [Integer that governs the ReaDing of _WFQ files]
Relevant internal variables:
... %qptn [Q-PoinT re-Normalized]
Input variables for experts:
... frzfermi [FReeZe FERMI energy]
... pawcross [PAW - add CROSS term in oscillator strengths]
... rf2_dkdk [Response Function : 2nd Derivative of wavefunctions with respect to K]
... rfmeth [Response Function METHod]
... rfuser [Response Function, USER-defined]
... scphon_supercell [Self Consistent PHONon SUPERCELL]
... scphon_temp [Self Consistent PHONon TEMPerature]
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tests/v6/Input: t62.in t63.in t89.in
tests/v7/Input: t43.in t44.in t45.in
tests/v8/Input: t07.in
t20.in
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