Dear Alexey If HSE would cost as much as DFT+U, I did surely use it for everything...:-) G.
On Friday 25 January 2013 01:35:13 Alexey Akimov wrote: > Dear Giuseppe, > > Thank you very much for sharing your experience. That is very deep > analysis, indeed. It is definitely a good suggestion (i find it useful for > myself, too :) ) I just wanted to point out that in general the DFT > results should be interpreted with care, especially in such a pathological > case when semiconductor is a metal computationally. It is good that +U > correction can help for this system, although it is somewhat empirical > approach. Perhaps, doing PBE0 calculations would be more straightforward > to apply and closer in spirit to the first-principles philosophy, although > more expensive. > > > ----- Original Message ----- > From: "Giuseppe Mattioli" <giuseppe.mattioli at ism.cnr.it> > To: "PWSCF Forum" <pw_forum at pwscf.org> > Sent: Thursday, January 24, 2013 6:40:28 AM > Subject: Re: [Pw_forum] Fail to predict semiconductor > > > Dear Alexey > I do not agree with your analysis. GGA is indeed affected by the well > known, bloody delocalisation error, which leads (among other, several, > painful problems) to an underestimation of the band gap of insulators and > semiconductors. This said, the Ti-->Zn substitution in the ZnO lattice > seems to be characterized by a quite peculiar behaviour that, in my > opinion, may be only partly accountable for the above delocalisation (or > double counting, self interaction, call it as you like...:-)) error. A > DFT+U correction, by the way, is often able to cure a vast majority of the > symptoms of delocalisation errors, but, like all drugs, must be carefully > used in the best way. A substitutional Ti atom has two excess electrons > with respect to the Zn one. In Iwan's calculation they are accommodated in > a hugely k-dispersed (i.e., highly delocalized) band which falls about 1.2 > eV above the valence band maximum at Gamma, and cross the conduction band > minimum in some regions of the Brillouin zone. A gap of about 3.0 eV, > obtained by "pushing down" the Zn 3d orbitals with a 7.0 eV U correction > and, therefore, by disentangling the narrow 3d band from the broader O 2p > band is quite similar to the optical 3.2~3.4 gap of ZnO, even if the Zn 4s > nature (and potential energy) of the conduction band minimum is nearly > unaffected by the correction. In my experience, a "conventional" behaviour > of a GGA calculation of Ti doped ZnO would be represented by one of the > following occurences > > a) the two excess electron populate the conduction band minimum of ZnO > > b) the two excess electrons are localized on atomic-like d orbitals of Ti > > The 5.5 eV correction applied to the Ti 3d shell should favour b), but the > actual results seem to be a curious mixing of a) and b). On the ground of > such an analysis, I would suggest to perform an nspin=2 calculation > because: > > a) Ti(3+) ions are often reported in the case of n-type doping of TiO2, at > variance with Ti(2+). I suspect that Ti cannot accommodate more than 1 > excess electron in a 3d-like small polaron. > > b) Iwan's results seems to suggest that the first excess electron could be > accommodated in a single- occupied, k-narrow, deep in the band gap Ti 3d > orbital, while the second one could be accommodated in the k-dispersed > conduction band minimum. > > c) If I'm right, I expect to be mentioned in the acknowledgment section of > Iwan's thesis...:-) > > Yours > > Giuseppe > > P.S. It is not really polite to mention it, but it may be useful to Iwan to > grab my recent publications on DFT+U calculations applied to TiO2 and > ZnO... > > On Wednesday 23 January 2013 21:53:37 Alexey Akimov wrote: > > Dear Iwan, > > > > The pure DFT is known to underestimate the band gaps, eventually making > > semiconductor material to appear as a metal in your calculations. This > > problem arises because of the double-counting in exchange terms. The > > problem solved with the hybrid functionals, such as PBE0. The GGA > > approximation and even +U correction terms provide only small improvement > > over LDA. So this may not be enough to make your system to be > > semiconductor (computationally). To summarize,the problem is inherently > > with the DFT methododology. > > > > Good luck, > > Alexey > > > > ----- Original Message ----- > > From: "Iwan Darmadi" <iwan_darmadi at rocketmail.com> > > To: "pw forum" <pw_forum at pwscf.org> > > Sent: Wednesday, January 23, 2013 12:50:35 AM > > Subject: [Pw_forum] Fail to predict semiconductor > > > > > > > > > > > > > > > > Dear all, > > > > > > > > I have calculated electronic structure of Ti doped ZnO in both GGA and > > GGA+U scheme. Both scheme predicts Ti doped ZnO is metallic. In contrary, > > Ti doped ZnO is well known as semiconductor experimentally. At first > > glance, I thought it was local minimum problem of DFT+U (like FeO problem > > in Mr. Himmetoglu's tutorial). Then I try to copy Mr. Himmetoglu's trick > > to override a "suspected" fully occupied orbitals of Ti. Sadly, nothing > > change, it's still a metallic. > > > > > > > > Now, I am confused whether this is a really local minimum problem or > > intrinsic limitation of DFT it self. > > > > > > > > Do anyone here have suggestions so I can get semiconductor Ti doped ZnO > > in the calculation ? > > > > > > > > Ps. > > > > I have also attached my input and output file. > > > > *** > > > > Iwan Darmadi > > Undergrad.Student - Department of Physics > > > > Universitas Indonesia > > > > _______________________________________________ > > Pw_forum mailing list > > Pw_forum at pwscf.org > > http://pwscf.org/mailman/listinfo/pw_forum -- ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ?gaux en droits. 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