[QE-users] Occupation calculation - epsilon.x code

[Andrea Ferretti]

Dear Anibal,

you are right, it seems calculation="occ" is documented but no longer 
there. As far as I understand, the reason is exactly that it is preferred 
to directly compute the quantity of interest and perform convergence 
checks on it (epsilon.x should be fast enough, though).

Regarding the anysotropy: as far as I remember epsilon.x does not 
implement symmetries, meaning that kpts need to span the whole BZ.
If you run a scf and a nscf calculation using pw.x and does not pay 
attention to this (meaning you have not set nosym=.T. noinv=.T.), kpts 
will be symmetrized and only the IBZ wedge will be sampled. In turn this 
can lead to spurious anysotropy (besides non-correct results).

hope it helps
Andrea

BTW: I have a newer version of epsilon.f90 contributed by Tae-Yun Kim 
(Seoul National University, South Korea) which fixes a number of these 
issues. Just haven't found the time to include it in the official distribution.

> 
> The epsilon.x manual (in the PP/DOC folder) shows the possibility of calculating occupations using the key "occ" within the epsilon.x. It is
> emphasized to be a good tool to analyze convergency against the broadening parameter and the k points sampling. However, the "occ"
> calculation is not implemented (at least in the version I'm using - 6.4). Such a calculation was implemented with other packages?
> 
> If not, is there a way of verifying the convergence other than explicitly changing the broadening and k points sampling?
> 
> I'm working with an AuAl alloy, trying to evaluate the dielectric function. Using epsilon.x I've got anisotropic behavior that I was not
> expecting for. Working with pure systems (Au and Al) I concluded that reducing conv_thr increases the epsilon.x output precision, returning
> the isotropic behavior of the dielectric function. Therefore, to the alloy (with 12 atoms in the cell), I increased both the conv_thr (1e-13)
> and k points (14 14 14). I still got the anisotropy. Should I go further (calculations with my actual computing power are becoming very time
> and memory consuming)? 
> 
> Thanks in advance!!
> 
> Anibal Bezerra
> The Federal University of Alfenas
> 
>

-- 
Andrea Ferretti, PhD
S3 Center, Istituto Nanoscienze, CNR
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322;  Skype: andrea_ferretti
URL: http://www.nano.cnr.it