[Pw_forum] Point group changes when I modify ecutrho

[Paolo Giannozzi]

See 5.0.0.19 and 5.0.0.20 in the user guide of PWscfs:
http://www.quantum-espresso.org/wp-content/uploads/Doc/pw_user_guide/node20.html#SECTION000600190000000000000
in particular this:

* they contain a fractional translation that is incompatible with the
FFT grid (see next paragraph). Note that if you change cutoff or unit
cell volume, the automatically computed FFT grid changes, and this may
explain changes in symmetry (and in the number of k-points as a
consequence) for no apparent good reason (only if you have fractional
translations in the system, though).
[...]

5.0.0.20 Warning: symmetry operation # N not allowed

This is not an error. If a symmetry operation contains a fractional
translation that is incompatible with the FFT grid, it is discarded in
order to prevent problems with symmetrization. Typical fractional
translations are 1/2 or 1/3 of a lattice vector. If the FFT grid
dimension along that direction is not divisible respectively by 2 or
by 3, the symmetry operation will not transform the FFT grid into
itself. Solution: you can either force your FFT grid to be
commensurate with fractional translation (set variables nr1, nr2, nr3
to suitable values), or set variable use_all_frac to .true., in
namelist &SYSTEM. Note however that the latter is incompatible with
hybrid functionals and with phonon calculations.

On Fri, Jun 10, 2016 at 1:58 PM, Bahadır SALMANKURT
<bsalmankurt at gmail.com> wrote:
> Dear All,
>
> I have been calculating structure parameter of ZrBeSi type crystal, whose
> point group is D6_h. When I take ecutrho as 500 Ry, everything is ok. But
> When I decrease ecutrho from 500 to 480 , its point group become D3_d even
> if crystal_sg is used.
>
> Why does such a thing happen?
>
> Thanks in advance
> --
>
> Bahadır SALMANKURT
> Ph.D. Candidate
> Department of Physics, Sakarya University, TURKEY
>
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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