[Pw_forum] problem with symmetry recognition

[Stefano de Gironcoli]

this is not correct.
if the code finds some symmetry, forces and stress components will be 
symmetrized accordingly  and vc-relax will keep it.

There may be two reasons why the code does not find a symmetry:

- the symmetry is not there... check your coordinates. Beware that you 
need to specify ALL the atomic positions of a given structure not just 
the inequivalent ones, hoping the code to fill the others.

- the symmetry is there but involves a fractionary translations that 
cannot be written as a multiple of the FFT grid spacing.  the code 
writes a message saying that the symmetry is discarded due to a problem 
with fractionary translation and says what the translation would be in 
crystal coordinates. one can manually set the FFT dimensions (nr1,nr2, 
and/or nr3) so that the translation is in register with the FFT grid and 
the code can accept it

HTH

stefano
-
Stefano de Gironcoli - SISSA and DEMOCRITOS



On 07/09/2011 07:58 PM, Vic Bermudez wrote:
> Hi Pascal,
>
> 	If I understand correctly, you're trying to optimize the unit cell while
> constraining 2 of the lattice constants to remain equal, as required by
> symmetry. I don't think that QE allows this. If you do a "vc-relax", what
> seems to happen is that symmetry is ignored, and all lattice constants are
> varied independently. I've attached a script written by a colleague that
> does repeated single-point runs for a range of lattice constants. You can
> then fit points near the total-energy minimum to find the exact minimum. Of
> course, this has to be done iteratively if more than one lattice constant is
> being optimized.
> 	Hope this helps.
>
> 	Best Wishes,
> 	Vic Bermudez
>
> Victor M. Bermudez
> Code 6876
> U.S. Naval Research Laboratory
> 4555 Overlook Ave., S.W.
> Washington, DC 20375-5347
>
>
>
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