[Pw_forum] change of crystal symmetry during vc-relax calculations with pressure

[saqib javed]

Dear Paolo,

Thanks a lot for a prompt reply. I am bit confused since i started with tetragonal structure (ibrav=6) so i would have imagined that a=b would have been enforced by the code in case symmetry is conserved during optimization ...
but just to understand the answer fully, you are saying that these results don't represent physical pressure-induced tetragonal to orthorhombic transition but just a numerical artificat ?

I will really appreciate a clarification..

cheers,
Saqib Javaid
NCP, Pakistan.


> From: paolo.giannozzi at uniud.it
> To: pw_forum at pwscf.org
> Date: Fri, 14 Jun 2013 10:43:36 +0200
> Subject: Re: [Pw_forum] change of crystal symmetry during vc-relax calculations with pressure
> 
> On Fri, 2013-06-14 at 07:18 +0000, saqib javed wrote:
> 
> > Can vc-relaxation at a given pressure with bfgs method allows for
> > change of crystal symmetry ?? 
> 
> no, but only the symmetry as found by the code is conserved;
> "accidental" symmetries, e.g. lattice vectors a and b having 
> equal length but no symmetry operation sending a into b, may 
> be easily lost due to numerical noise.
> 
> Paolo
> -- 
>  Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
>  Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>  Phone +39-0432-558216, fax +39-0432-558222 
> 
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