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

[saqib javed]

Dear Paolo,
Actually lattice parameters obtained for high pressures ( 25, 30, 35 GPa) are the same if initial structure is either tetragonal or orthorhombic. Furthermore the initial symmetry for both cases as written at start of output reads the same
2 Sym. Op. (with inversion)
Thus perhaps change from tetragonal to orthorhombic structures at high pressure may not represent symmetry breaking ?? And results obtained are physicall meaningful i.e., tetragonal phase is unstable at high pressures in favor of orthorhombic phase. Plz correct me if you see anything wrong in my reasoning.
cheersSaqibNCP, Pakistan

> From: paolo.giannozzi at uniud.it
> To: pw_forum at pwscf.org
> Date: Fri, 14 Jun 2013 12:34:57 +0200
> Subject: Re: [Pw_forum] change of crystal symmetry during vc-relax calculations with pressure
> 
> On Fri, 2013-06-14 at 09:13 +0000, saqib javed wrote:
> 
> > 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 ...
> 
> symmetry IS conserved during optimization, apart from unfortunate cases,
> but only the symmetry of the starting configuration, as found by the
> code. Look at which symmetries are found by the code in the starting
> configuration.
> 
> > you are saying that these results don't represent physical
> > pressure-induced tetragonal to orthorhombic transition but just a
> > numerical artificat ?
> 
> I don't know. Likely: it is a 0.3% difference. Do you trust it?
> I wouldn't. At least, not without some additional evidence.
> 
> P.
> -- 
>  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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