this is not correct. if the code finds some symmetry, forces and stress components will be symmetrized accordingly and vc-relax will keep it.
There may be two reasons why the code does not find a symmetry: - the symmetry is not there... check your coordinates. Beware that you need to specify ALL the atomic positions of a given structure not just the inequivalent ones, hoping the code to fill the others. - the symmetry is there but involves a fractionary translations that cannot be written as a multiple of the FFT grid spacing. the code writes a message saying that the symmetry is discarded due to a problem with fractionary translation and says what the translation would be in crystal coordinates. one can manually set the FFT dimensions (nr1,nr2, and/or nr3) so that the translation is in register with the FFT grid and the code can accept it HTH stefano - Stefano de Gironcoli - SISSA and DEMOCRITOS On 07/09/2011 07:58 PM, Vic Bermudez wrote: > Hi Pascal, > > If I understand correctly, you're trying to optimize the unit cell while > constraining 2 of the lattice constants to remain equal, as required by > symmetry. I don't think that QE allows this. If you do a "vc-relax", what > seems to happen is that symmetry is ignored, and all lattice constants are > varied independently. I've attached a script written by a colleague that > does repeated single-point runs for a range of lattice constants. You can > then fit points near the total-energy minimum to find the exact minimum. Of > course, this has to be done iteratively if more than one lattice constant is > being optimized. > Hope this helps. > > Best Wishes, > Vic Bermudez > > Victor M. Bermudez > Code 6876 > U.S. Naval Research Laboratory > 4555 Overlook Ave., S.W. > Washington, DC 20375-5347 > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110709/26f935ad/attachment-0001.htm
