[Pw_forum] About valence bands maximum

[roma]

Dear Ian Haiping,

it is true that seldom papers about defects discuss this point, and I am
probably guilty of this too. My understanding is that your expression
for the valence band top of the defect is correct, but then it depends
on how you use it. What has been omitted according to the person that
told you this? 
The crucial quantity is [<V(D,q)> -<V(0)>] (let us call it
DeltaV(D,q) ). Long ago I did a small program to calculate the so called
macroscopic average of the potential (Balderesci, Baroni,Resta, PRL v61,
p734, 1988) in the supercell: it was ok for a defect in a simple metal
(I could find the "bulklike" region) but for the system I was interested
in, SiO2, the oscillations of the macroscopic average along the
supercell made it useless. Supercell too small? Problems due to the
symmetry of my supercell? I adopted a different strategy, I determined
the energy shift that maximises the overlap (or minimises the square
differences) of the density of states of the defected and perfect
crystal. This shift is for me DeltaV(D,q). 
This, according to my experience, can be unambiguosly determined as long
as the relaxations are good (and k-point sampling is sufficient).
Then another question is to which extent the DeltaV term corrects for
the image interaction of charged defects, on which I would appreciate
also the feedback from others on the list.
Best regards,

Guido



 

On thursday April 12 Ian Haiping wrote:

>    I want to determin  the transition enery of a defect level.  It is
>important to align the valence bands maximum of different systems under
>investigation. Many works  has claimed valence bands maximum alignments
>were performed but no technique details are given. 


>For my understanding, 


>I thougt the valence bands maxium of charge defect systems could be
>determined by aligning its average potential 


>to pure perfect system's potential, e.g  \epsilon_{VBM}.(Defects,q) =
>\epsilon_{VBM}(0) + [<V(D,q)> -<V(0)>].   But some person told me  such
>expression has omitted something.    So it really confused me much. As
>far as i know,  the potential of system could be only determined  to
>some constant . i thought the alignment of average potentials of two
>systems could take into account this arbitrariness.. I am not very
>certain about this  VBM alignment right now   Would you give me some
>comments and clarify my understanding ?  


> 
-- 
Guido Roma <roma${at}cea.fr> -- CEA-Saclay - DEN/DMN/SRMP Bat.520/13
Phone: [+33]-1-69081857 -- Fax: ...6867 -- Mobile: [+33]-6-20069085