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
