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"
> To: "PWSCF Forum"
> 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"
> > To: "pw forum"
> > 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.
