[Pw_forum] Questions about ionic & cell relaxation.
h.zhang at ifw-dresden.de
Sat Jan 19 19:13:08 CET 2008
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
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
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.
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).
> > 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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