[QE-users] Keep the symmetry during a calculation

[Paolo Giannozzi]

On Wed, Sep 26, 2018 at 2:08 PM, Carina Backtorp <carinabacktorp at hotmail.com
> wrote:

>
> As an example, if one places ONE  water molecule in a large cubic box and
> runs a vc-relax calculation in QE with ibrav = 1, i.e. cubic symmetry, then
> QE does a symmetry analysis concluding that the crystal symmetry is not
> ibrav 1 and therefore does the calculation with the correct symmetry.
>

exactly: at the end of the calculation, your cell may no longer be cubic

 If so, is there a key word in QE that I can include into the input file so
> that QE gives the obtained corrected symmetry in the output file?
>

no

>
> Also, is it possible to give QE a constraint in the input file so that it
> keeps the symmetry that was given? For example: even if the crystal
> structure does not have the cubic symmetry according to QE, can one still
> keep all three box lengths equal during a vc-relax calculation?
>

option cell_dofree='volume' should keep the three axis as they are, varying
only the volume

Paolo

>
> Thank you for all help!
> Kind regards,
> Carina
>
> ------------------------------
> *From:* Paolo Giannozzi <p.giannozzi at gmail.com>
> *Sent:* Wednesday, September 19, 2018 3:44 PM
> *To:* Quantum Espresso users Forum
> *Cc:* Carina Backtorp
> *Subject:* Re: [QE-users] Keep the symmetry during a calculation
>
> On Sat, Sep 15, 2018 at 10:09 AM, Carina Backtorp <
> carinabacktorp at hotmail.com> wrote:
>
>
> 1) When doing an vc-relax calculation in espresso, I expected that once
> the symmetry was given (ibrav=1) espresso should keep the symmetry during
> all the calculation
>
>
> it does. Note however that the symmetry of the lattice is just part of the
> story.  QE finds the starting symmetry of the crystal, and this is what is
> kept during the run (although occasionally, numerical noise and poor
> convergence may lead to the loss of the original symmetry). If the starting
> symmetry is not cubic, the final cell may lose its original cubic aspect.
> Also note that QE uses its own criteria for symmetry, that may differ from
> criteria used by other codes, so something that QE deems cubic may be
> classified as orthorhombic by a pickier code
>
> Paolo
>
>
>
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>
>
>
> --
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> <https://maps.google.com/?q=Udine,+via+delle+Scienze+208,+33100+Udine,+Italy&entry=gmail&source=g>
> Phone +39-0432-558216, fax +39-0432-558222
>
>


-- 
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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