[Pw_forum] Questions about ionic & cell relaxation.

[Hongbin Zhang]

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)?

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
> >
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>
>
> --
> ---------------------------------------------------------------------
> 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
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