[Pw_forum] Fw: defect formation energy

[Gabriele Sclauzero]

Duy Le wrote:
> if the defect concentration is too hight, when you do vc-relax the 
> lattice parameter will be re-optimized. In reality, the defect 
> concentration is about zero.

I must admit that I have no experience in computing defects, but I agree with the above 
comment. If you want to study very low concentrations you should take care of converging 
your results with the supercell size (keeping lattice parameters "freezed", i.e. not 
reoptimizing them after defect formation) and add Makov-Payne-like corrections if you are 
dealing with charged defects.
If you really want to study very high concentrations and take into account changes in the 
volume of the crystal, you should maybe consider formation enthalpy, not simply formation 
energy.

HTH

GS


> 
> On Fri, Sep 18, 2009 at 8:43 AM, xirainbow <nkxirainbow at gmail.com 
> <mailto:nkxirainbow at gmail.com>> wrote:
> 
>     Dear:
> 
>     When you calculate without vc-relax, you deal with a point defect.
>     When you use vc-relax, you change the crystal parameters. That is to
>     say, you deal with a new kind of crystal, not a point defect.
>     It is just my own point of view ：）
> 
> 
> 
> 
>         Ali Kazempour
>         Physics department, Isfahan University of Technology
>         84156 Isfahan, Iran. Tel-1: +98 311 391 3733
>         Fax: +98 311 391 2376 Tel-2: +98 311 391 2375
> 
>         --- On *Fri, 9/18/09, ali kazempour /<kazempoor2000 at yahoo.com
>         <mailto:kazempoor2000 at yahoo.com>>/* wrote:
> 
> 
>             From: ali kazempour <kazempoor2000 at yahoo.com
>             <mailto:kazempoor2000 at yahoo.com>>
>             Subject: defect formation energy
>             To: forum at democritos.it <mailto:forum at democritos.it>
>             Date: Friday, September 18, 2009, 5:31 AM
> 
> 
>             Hi
>             for study of point defect ,  i do relax calculation and
>             obtain positive defect formation energy while by vc-relax i
>             gain negative. ?
>             what is the difference between positive and negative
>             formation energy? waht is the physical reasons?
>             thanks a lot
>             Ali Kazempour
>             Physics department, Isfahan University of Technology
>             84156 Isfahan, Iran. Tel-1: +98 311 391 3733
>             Fax: +98 311 391 2376 Tel-2: +98 311 391 2375
> 
> 
> 
> 
> 
>     -- 
>     ____________________________________
>     Hui Wang
>     School of physics, Nankai University, Tianjin, China
> 
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> 
> 
> 
> -- 
> --------------------------------------------------
> Duy Le
> PhD Student
> Department of Physics
> University of Central Florida.
> 
> 
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