[QE-users] Symmetry Considerations

[Michael Mehl]

Willem,

Add the command

use_all_frac=.true.

to &system.  IIRC, this adjusts the FFT grid to be commensurate.

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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[image: USNA_Gold_Seal.png]

Michael J. Mehl, Ph.D.

Kinnear Chair in Physics

The United States Naval Academy

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mmehl at usna.edu

Library of Crystallographic Prototypes <http://aflow.org/CrystalDatabase/>
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