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

[Peter Blaha]

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
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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