[Pw_forum] point charge

[Stefano Fabris]

Dear Ali,

you have to give a deeper thought on your concept of "putting the  
charge in the right position". In standard calculations this is a  
result not an input. As a specific example in the defects you mention,  
think at ionic systems where the charge localization around a given  
defect is mainly (but not only) controlled by the Madelung potential.  
Note that in these systems a charged defect does not necessarily imply  
a charged supercell. But you are right, there are cases in which  
different charges can be associated to a given defect (think for  
example at F centers that can exists in several charge states), and  
here you do need charged supercells. QE can deals with these cases by  
specifying the number of electrons in the system, and adding a  
compensating uniform background. Note that in these cases,  
thermodynamic arguments allow for predicting which is the most stable  
defect in given environment conditions. There are several studies  
about this issues (including the screening), the first ones I found in  
my records are these for ionic systems  PRB 68, 085110 (2003) and PRB  
59, 797 (1999); and this one for semiconductors: PRL 67, 2339 (1991).  
These, and the references in within, should give you enough concepts  
(and keywords) to performs more focused literature research.

Yours sincerely,
			Stefano
On 19 Aug 2009, at 10:39, ali kazempour wrote:

> Hi
> I want to know if  espresso  can do charged vacancy calculation or  
> not? I know about isolated atoms but I am not sure about big bulk  
> supercell?
> If it can, how we can put the charge on the right position of the  
> vacancy(which constraint should we introduced)?
> another question, Could anyone introduced me some reference about  
> screening effect around the point defect?
> 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
>
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---
Stefano Fabris
Theory at Elettra Group
CNR-INFM DEMOCRITOS National Simulation Center
c/o Sincrotrone Trieste - SS14, Km 163,5 Basovizza, I-34012 TRIESTE
website: www.democritos.it/the-group
email: fabris at democritos.it    tel: +39 040 375-8735    fax: -8776
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