[QE-users] Symmetry Considerations

[Paolo Giannozzi]

On Thu, Jan 10, 2019 at 9:09 PM Michael Mehl <mmehl at usna.edu> wrote:

Add the command
>
> use_all_frac=.true.
>
> to &system.  IIRC, this adjusts the FFT grid to be commensurate.
>

not exactly so (IIRC): until the last stable version, use_all_frac=.true.
allows the use of nonsymmorphic operations even if the real-space FFT grid
is not commensurate (this works except for phonons and hybrid functionals).
This will change in the next stable version, the FFT grid will be adjusted
to be commensurate by default (use_all_frac=.false.), not adjusted if
use_all_frac=.true.. In both cases, nonsymmorphic operations will work, but
in the latter case, phonons and hybrid functionals will not.

Paolo

Alternatively, you can set the values of the FFT grid (nr1, nr2, nr3) by
> hand.  Depending on the fractional translation, making these integers
> multiples of 2, 3, 4, or 6 should solve your problem.
>
> Note the warning in the PW documentation about when you shouldn't set
> use_all_frac
>
>
> On Thu, Jan 10, 2019 at 10:30 AM Offermans Willem <
> willem.offermans at vito.be> wrote:
>
>> Dear Quantum Espresso friends,
>>
>> All my systems of interest have S2-symmetry, i.e. there is an inversion
>> centre present.
>> In one of my previous mails, I complained about loosing symmetry. I
>> calculate my systems
>> sequently with increased accuracy. I clearly started with a system with
>> an inversion centre.
>>
>> There was a message in the output of the first calculation:
>>
>>      2 Sym. Ops., with inversion, found ( 1 have fractional translation)
>>
>> I realise now that I did not read this message carefully. I missed the
>> last part about fractional translation.
>> What does it mean?
>>
>> At some point in the sequence of calculations, the output contained the
>> following message about symmetry:
>>
>>     No symmetry found
>>
>> I was surprised about the lost of symmetry. Is it related to the
>> fractional translation?
>>
>>
>> Today I put a lot of effort to impose the symmetry. I read one part of
>> the system from output and generated
>> the other part by inversion.
>>
>> Now the output contained the following message:
>>
>>     No symmetry found
>>           (note:  1 additional sym.ops. were found but ignored
>>            their fractional translations are incommensurate with FFT grid)
>>
>> How can I bring the fractional translations of the sym.ops in
>> commensurate mode with FFT grid?
>>
>>
>> Met vriendelijke groeten,
>> Mit freundlichen Grüßen,
>> With kind regards,
>>
>>
>> Willem Offermans
>> Researcher Electrocatalysis SCT
>> VITO NV | Boeretang 200 | 2400 Mol
>> Phone:+32(0)14335263 Mobile:+32(0)492182073
>>
>> Willem.Offermans at Vito.be
>>
>>
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>
>
> --
>
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>
> Michael J. Mehl, Ph.D.
>
> Kinnear Chair in Physics
>
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>
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>
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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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