[Pw_forum] Questions about ionic & cell relaxation.

[Axel Kohlmeyer]

On 1/19/08, Hongbin Zhang <h.zhang at ifw-dresden.de> wrote:
>
> Dear Nicola and all users,
>
> Thanks for you comments. But there're still something unclear for me:
>
> 1) if the symmetry will be kept, then how can I break the symmetry if
> necessary?
>
> 2) I think there is a Keyword "nosym" in the namelist "&SYSTEM", is it
> somehow related to structure relaxation? Since in example 03 for Al(001)
> slab, when relaxeing, "nosym = .true" is used.
>
> 3) In "ATOMIC_POSITIONS" catalog, it's possible to set "if_pos" terms,
> which will make forces along some directions to be zero. How should I
> combine these options with "nosym" option?
>
> Maybe it would be more clear if I show you my intended calculations:
>
> basically I want to optimize atomic positions of a metal/insulator/metal
> supercell only along z direction (x&y coords won't change), and there're
> two steps:
>
> 1) optimization with mirror symmetry along z direction
>
> 2) optimization without mirror symmetry along z direction
>
> So if the original supercell have the z-mirror symmetry, how to break it
> if relaxation will keep all symmetry pwscf find (nicola)?

if you want to break the symmetry, just move 1-2 atoms a
bit (or a bit more), so that symmetry is broken (enough).

cheers,
    axel.

>
> Many thanks for your comments and suggestions.
>
> Best wishes,
>
> Hongbin
>
>
>
>
>
>
>
>
> On Thu, 17 Jan 2008, Nicola Marzari wrote:
>
> > Hongbin Zhang wrote:
> > > Dear all pwscf users,
> > >
> > > I want to do some ionic relaxations, but there's one question unclear:
> > >
> > > Will the relaxation lower the original symmetry?
> >
> >
> > Ionic forces will satisfy all the symmetry operations that PWSCF
> > has found, and so relaxations will preserve those symmetry operations.
> >
> > Generally speaking, you should be fine - if anything, one needs
> > to be careful of the opposite problem, when the ionic ground state has
> > less symmetry than the initial configuration (since the relaxations
> > do not break the symmetry).
> >
> >                       nicola
> >
> >
> >
> > >
> > > The reason why I ask this is that I want to optimize the cell and ionic
> > > positions with some constraints, say, to keep the mirror symmetry along
> > > z direction of the unit cell.
> > >
> > > Any suggestion and comment are welcome. Thank you in advance.
> > >
> > > Best wishes,
> > >
> > > Hongbin Zhang
> > >
> > > _______________________________________________
> > > Pw_forum mailing list
> > > Pw_forum at pwscf.org
> > > http://www.democritos.it/mailman/listinfo/pw_forum
> >
> >
> > --
> > ---------------------------------------------------------------------
> > Prof Nicola Marzari   Department of Materials Science and Engineering
> > 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> > tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
> > _______________________________________________
> > 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
>
>


-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
  Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.