On Wed, Sep 26, 2018 at 2:08 PM, Carina Backtorp <[email protected] > 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 <[email protected]> > *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 < > [email protected]> 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 > > > > _______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users > > > > > -- > 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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