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

[saqib javed]

Dear Mohnish,
I am familiar with the concepts regarding crossover of enthalpy as you just mentioned. But what if i start with tetragonal structure but end up with something close to orthorhombic ? The calculated enthalpy will belong to which particular symmetry ? I suspect ( not sure ) that such thing may also happen when  the starting crystal structure is unstable for a target pressure.
Many thanks to Paolo and you for the clarifications,
cheersSaqib
Date: Fri, 14 Jun 2013 16:54:23 +0200
From: mohnish.iitk at gmail.com
To: pw_forum at pwscf.org
Subject: Re: [Pw_forum] change of crystal symmetry during vc-relax calculations with pressure

Dear Saqib,

The number of symmetry operations may be the same but the 'kind' of symmetry may be different.

The way to study the pressure induced phase transition is to plot the enthalpy as a function of pressure both the systems and look for the crossover. Geometry optimization doesn't convert the one type of crystal to the other. So you have to do the calculations for both of your system. 


At the transition pressure the enthalpy of the dominant phase will be lower. I hope it helps.


On Fri, Jun 14, 2013 at 3:09 PM, saqib javed <sqbjd at hotmail.com> wrote:




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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-- 
Regards,
MOHNISH,
-----------------------------------------------------------------

Mohnish Pandey,
PhD Student,
Center for Atomic-scale Materials Design,
Department of Physics,
Technical University of Denmark
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