[Wien] non-corrolated orbitals and GGA+U approximation

[tran at theochem.tuwien.ac.at]

Just to be more specific:
yes, the exact DFT potential would not give the correct gap. However,
this statement applies for the case of orbital-independent potentials
(like MBJLDA), while LDA+U is an orbital-dependent potential.

On Wed, 16 May 2012, Peter Blaha wrote:

> With LDA+U you can do almost anything by choosing some parameters (try
> negative U,....)
> and thus, if you know the experiment, you can play around and vary U of some
> atoms and states
> until you get the desired result (you could probably also use a larger U on Zn
> and a smaller
> one on O and get the same gap).
> 
> The question is always: is it physical reasonable and justified, or is it just
> a dirty fix
> to reproduce some experimental value.
> 
> The answer to this question is not that simple. For sure, in GGA also the O-p
> states are not treated
> 100% correctly, but they should be "better" than the Zn-d states. Thus,
> personally I don't like
> a calculation with U(O-p)=1.1 Ry.
> 
> In addition, remember that even the "exact DFT functional" (if we ever get to
> know it), would NOT
> give you the correct gap.
> 
> Am 16.05.2012 11:23, schrieb masoud majidiyan:
> > Dear pro.Balha,
> > we want to know if it is reasonable to use GGA+U for the
> > non-corrolated orbitals like p orbitals?
> > we have used GGA+U for O-2p and Zn-3d orbitals with Ud=0.98 Ry and
> > Up=1.1Ry and in this case we obtained the band gap of ZnO ( wurtzite
> > structure) 3.03 eV , which is very close to the experimental
> > results.Also, we have used MBJLDA approximation and  obtained the
> > energy gap 2.2 eV, which is quite smaller than the value obtained
> > using GGA+U approximation, although we expected to get  an exact
> > result with MBJLDA!
> > thank you very much in advance
> > `
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> 
> -- 
> 
>                                       P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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