[Wien] Band Gap problem

[Javad Hashemifar]

> I have calculated band structure of GaN in wz phase under GGA(96) 
> approximation with band gap Eg=2.2eV,while the pseudopotential and 
> Experimental data show that it is equals to 3.5eV.Who can help me in 
> some simple terms that what is the reason of this main difference 
> between these calculations.Of course,I have calculated band structure of 
> GaN with other approximations( LSDA,GGA(91) ),that is not affect.

Dear Behrooz Rezaei,
I am not expert in this field, but it seems that usual LDA based
Exchange-correlation approximations are not appropriate for calculation of the
band gap of semiconductors. Peoples are developing some other approximations
like (Exact-Exchange method for better treatment of the band gap of
semiconductors).
In wien2k code, as far as I know, there is an option for exchange correlation
potential called Engel-Vosko approximation (PRB47, 13164) that seems to leads
to better band gaps rather than other approximations (LDA and GGA). In order to
use this approximations you have to use one of the switches 15, 25 or 26 as
VXC-SWITCH in case.in0 input file. I think switch 25 is better but I am not
sure. It would be nice if some of expert wien users help more about using this
exchange-correlation approximation. 
Best regards,
Javad Hashemifar



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Javad Hashemifar                                      phone : ++49-30-8413 4804
Permanent : Physics Department, Isfahan University of Technology, Isfahan, Iran
Current : Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany
e-mails:  hashemi at fhi-berlin.mpg.de          &          hashemifar at ph.iut.ac.ir
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