[Wien] Band Gap problem

[Rezaei Behrooz]

Dear all wien users

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.
Thank you very much in advance.
best wishes,
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Behrooz Rezaei                              Email: b.rezaei at tabrizu.ac.ir
Research Institute for Applied Physics and Astronomy 
Tabriz University                               phone: +98-411-3393008 
Tabriz, Iran                                      Fax: +98-411-3347050 
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