cell_dofree ? i never used it but it may do something similar to what you look for. stefano
On 07/09/2011 10:46 PM, giuseppe.mattioli at mlib.ism.cnr.it wrote: > Dear Stefano, dear all. > Ok, but suppose that I would like to put a (quite useful) constraint > like: "I say that the lattice is cubic because I know that it MUST be > cubic (or tetragonal or monoclinic or so...), than KEEP IT CUBIC while > performing a vc-relax, irrespective of the stress tensor! Change the > volume, change the a/c ratio (if the lattice is, e.g., tetragonal) but > retain the space group. I often have to perform a (boring) multi > dimensional fit by changing the lattice parameters by hand on a grid > near to the "expected" minimum... > I do not know exactly if Pascal's question was similar to mine; > nevertheless I hope this can help... > > Yours > > Giuseppe > > > Quoting Stefano de Gironcoli<degironc at sissa.it>: > >> 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 > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum
