[Pw_forum] HYBRID XC not allowed in non-scf calculations

[Layla Martin-Samos]

Dear Eduardo, ecut_ws is a real space cutoff on the wigner Seitz cell for
coulomb. We (Giovanni Bussi Amdrea Ferretti and myself) have developped
this for SaX (GW calculations and BSE). The issue was that for strongly
anysotropic cells the coulomb potential is not behaving any more as the FFT
of 1/q**2 cloes to q=0. Usual techniques like Baldereschi-massidda and
others "correct " only q=0, and this is not enough. So the ecut_ws is a
cutoff that says to the code since how big q+G it has to correct coulomb:
the more anysotropic the more you have to have big this ecut_ws, but it
will take more time. The method is similar to Martyna-Tuckermans one withe
the difference that we just cutoff the fock operator (this is alsways valid
as the overlaps goe to 0 when r-r' -> infinity). If you are interested I
have a presentation, and we can discuss these questions by skype.
Unfortunatelly, we did nerver write the corresponding paper (always to much
work to do). But it is planned to do it one day.


hope this can help

cheers

Layla
2012/10/29 Eduardo Ariel Menendez Proupin <eariel99 at gmail.com>

> Hi Layla and everybody,
>
> Let us forget the nscf calculation for the moment, as my scf calculation
> was probably not fine.
> I have a strongly anisotropic unit cell, and large enough not to dare to
> attempt to build a quasi-cubi one, that would have about 360 atoms. For
> anysotropic cell,  INPUT_PW instructs to use
>
>                       exxdiv_treatment = 'vcut_ws',
> for this to work one must use
>                           x_gamma_extrapolation = .false.,   (b.t.w., it
> is not mentioned at INPUT_PW.txt )
> then I have to set the value
>                           ecutvcut = X ,  ! which is 0.7 Ry in the Si
> example.
>  so, what is the method followed by using vcut_ws ,  and how to set
> ecutvcut ? Are low or high values of ecutwfc more or less accurate or more
> or less expensive? Is it the method given by Sorouri, Folkes and Hine in
> JCP124, 064105 (2006)?
>
> Thanks
> --
>
>
> Eduardo Menendez Proupin
> Departamento de Química Fisica Aplicada
> Facultad de Ciencias
> Universidad Autónoma de Madrid
> 28049 Madrid, Spain
> Phone: +34 91 497 6706
>
> On leave from: Departamento de Fisica, Facultad de Ciencias, Universidad
> de Chile URL: http://fisica.ciencias.uchile.cl/~emenendez
>
>
>
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