On Mon, Oct 2, 2017 at 4:25 PM, JAY Antoine <[email protected]> wrote:
> I would like to perform a vc-relax that conserves the point group of each > atom and the symmetry of the cell. > vc-relax conserves the starting symmetry of the crystal. There is no way to conserve a symmetry that is not there at the beginning of the structural optimization. > For example, in a trigonal cell, ibrav 5 imposes the symmetries: > v1 = a(tx,-ty,tz) > v2 = a(0,2ty,tz) > v3 = a(-tx,-ty,tz) > but these symmetries seem to be broken by the relaxation, > see above: symmetries that were present in the starting structure are not broken. If a "symmetry" is "broken by structural relaxation", it wasn't a symmetry. so that at the end of the relaxation, v2(1) is not exactly null and v1(1) > is not exactly -v3(1), > or for the atomic positions, a coordinate (x, x, x) alat becomes (x, 0.99 > x, 1.001x ). > > > ?? Is it possible to impose that these constraints on the unit-cell and on > the atomic positions stay fix during the relaxation process ?? > > This is a major issue for the use of these relaxed values in a second step > for phonons for example, because it will then not detect the symmetries. > I think it could be a good idea to add into the vc-relax output file a > lign that gives the corresponding celldm(i) and not only the > CELL_PARAMETERS. > I also think that it is a good idea, but it takes a bit of time and effort, that becomes a byte, or even a word (32 or 64 bits) if one wants to write a piece of code that works perfectly in all cases Paolo > In fact, in the scf before the phonon use, we would like to have ibrav =/0 > and celldm in state of ibrav 0 and quasi symmetric CELL_PARAMETERS... > > Best regards, > > Antoine Jay > > > > > > > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- 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
_______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
