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 > ` > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien -- 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/ --------------------------------------------------------------------------
