[Pw_forum] Questions about ionic & cell relaxation.

[Paolo Giannozzi]

On Jan 19, 2008, at 19:13 , Hongbin Zhang wrote:

> 1) if the symmetry will be kept, then how can I break the symmetry if
> necessary?

try with a hammer :-)
Seriously: move one or a few atoms away from their symmetric positions

> 2) I think there is a Keyword "nosym" in the namelist "&SYSTEM", is it
> somehow related to structure relaxation?

what the "nosym" option (a very misleading name in my opinion) does
is explained in detail and exactly in the INPUT_PW file. Do not use it
unless you understand it.

> 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?

these two options do not interact at all

> 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)?

move just one atom from (x,y,z) to (x,y,z+dz), with dz > 10^(-5) 
*lattice parameter

Paolo
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Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222