[QE-developers] fractional translations

[Paolo Giannozzi]

Hi Davide, there is no need for group theory and symmetry here, it's a
simple programming matter. Previously, there were two versions of the
fractional translation: the original one, "ftau", integer, contained
fractional translations multiplied by FFT dimensions nr1,nr2,nr3; "ft",
real, contained the true fractional translations in crystal axis. "ftau"
was introduced (many years ago) because it provided real-space FFT indices
for the single place where fractional translations were actually used:
real-space symmetrization. Things have become more complex since. The
existence of two copies of the same variables had become confusing.
Moreover, I think that symmetry-related variables should be disentangled
from FFT grids.

Just use "ft", or introduce a local "ftau" variable if convenient. An
example: look for ftau in PW/src/exx_base.f90

Paolo

On Tue, Dec 17, 2019 at 12:04 PM Davide Ceresoli <davide.ceresoli at cnr.it>
wrote:

> Dear QE developers,
>      I've noticed the following change in 6.5:
>
> * fractional translations "ftau" in FFT grid units no longer existing as
> global
> variables: replaced by "ft", in crystal axis, computed locally where
> needed (in
> real-space symmetrization only)
>
> This change affects the symmetrization of chemical shifts of QE-GIPAW.
>
> I've never been good in group theory and symmetry. Can you help me to
> recover
> the behavior of QE pre-6.5?
>
> Thank you in advance.
>
> D.
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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