[Pw_forum] symmetries during a vc-relax

Paolo Giannozzi p.giannozzi at gmail.com
Mon Oct 2 21:43:56 CEST 2017

On Mon, Oct 2, 2017 at 4:25 PM, JAY Antoine <Antoine.JAY at isae-supaero.fr>

> 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

> 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

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

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


> 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
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> Pw_forum at pwscf.org
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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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