[Pw_forum] problem with symmetry recognition

[Giuseppe Mattioli]

Dear Stefano, dear all.
cell_dofree is useful when, for instance, you want to keep at least 
orthorombic a starting cubic cell, because you can force alpha, beta and 
gamma to be pi/2 (cell_dofree='xyz'). But there is no way (at least, no way 
that I'm aware of) to change all a, b and c and, at the same time, keep 
a=b=c. Suppose that you want to optimise a CoO rocksalt antiferromagnetic 
supercell: you need to differentiate the starting spin up and spin down Co 
atoms (Co1 and Co2). This lowers the symmetry, and the supercell becomes 
tetragonal along the simulation. I could provide further examples, and I 
think that a cell_dofree flag which allows one to retain the initial ibrav 
would be quite useful. I do not like to ask for things that I cannot 
implement by myself, but I may suggest that it could be a useful 
improvement...

Yours

Giuseppe

On Sunday 10 July 2011 23:20:08 Stefano de Gironcoli wrote:
> If the symmetry is recognized vc-relax should keep it
> if it is not there vc-relax cannot produce it unless the minimum the
> system converges to has that symmetry.
> there  are codes (I recently discovered GULP) that can accept input
> structure defined by the positions of the irreducible atoms + space
> group definition...
> pw.x does not have this capabiloity but if you generate the full list of
> coordinates with one of such codes then pw.x should be able to recognize
> the symmetry and keep it during relaxation.
>
> stefano
> -
> Stefano de Gironcoli - SISSA and DEMOCRITOS
>
> On 07/10/2011 09:09 PM, giuseppe.mattioli at mlib.ism.cnr.it wrote:
> > Dear all
> > I was thinking again about keeping the space group symmetry fixed
> > along a vc-relax calculation. Actually, I did not directly ask my main
> > question...:-) Is there a way to put the above constraint which I am
> > not aware of?
> > Yours
> >
> > Giuseppe
> >
> > ISM-CNR
> > Italy
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum



-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM)          
   Tel + 39 06 90672836 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>