[Pw_forum] epsilon.x

[marsamos at democritos.it]

epsilon.x calculate the dielectric function within the RPA 
approximation (no excitonic effects) without including local field 
effects and without including the commutator of H with r (contribution 
from the nonlocal part of the pseudo). more complete calculation of 
dielectric functions, can be made using SaX or yambo, or almost all the 
GW package available around. The equations on how to "implement", 
non-local fields or the inclusion of the non-local part of the pseudo 
can be found in old GW papers (REv. Mod. Phys. Onida, Reining ...).

Quoting Paolo Giannozzi <giannozz at democritos.it>:

> Emine Kucukbenli wrote:
>
>> It is clearly stated there that "the non-local contribution due to the
>> pseudopotential is neglected. Actually the correction to the matrix
>> element that take into account the non-local part of the Hamiltonian
>> is not implemented."
>> So to develop the code for USPP you need to find a way to calculate &
>> add these corrections, I guess.
>
> it is not sufficient. For norm-conserving PP, there is a nonlocal
> contribution to the matrix elements <\psi_i| r | \psi_j> (i /= j)
> that is not accounted for in epsilon.x . This is the meaning of
> the above statement.
>
> For USPP, additional terms have to be added, since only the smooth
> term of |\psi_i> (the one expanded into plane waves) is available.
>
> The correct calculation of matrix elements is actually performed
> in the phonon code and could be in principle imported into
> epsilon.f90.
>
> P.
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy
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