[Wien] Confusion in Selecting proper value of Hubbard potential.

[tran at theochem.tuwien.ac.at]

You can try larger U to see what happens, but values above 10 or 12 eV
start to be unphysical in solids. Also, the more the top of the valence
band or bottom of conduction band has an important contribution not coming
from the d or f electrons (e.g., oxygen-p), the less GGA+U will be
efficient to adjust the band gap. GGA+U does not always solve problems in
stronlgy correlated systems. As mentioned by Hong Jiang, you should maybe
consider other methods.

F. Tran

On Sat, 20 Jul 2013, Zaid wrote:

> Dear Sir H. Jiang
> 
> I implemented GGA+U to my considered compound, keeping in mind that GGA+U
> is valuable for compounds having strongly correlated system (like in d and
> f orbitals). But My problem is regarding the selection of U value.
> 
> Experimental band gap for my compound is 2.0eV. The maximum band gap on
> increasing U value for my calculation is 1.117eV at 10 eV. So, should I
> increase U value beyond 10eV ? Untill now from my literature review, the
> value of U is less than 10 eV.
> Secondly, if suppose a compound has not been studied experimentally then
> how we choose calue of U ?
> 
> Kindly guide.
> 
> Best regards
> Zaid
> 
> 
> On Sat, Jul 20, 2013 at 2:31 PM, Hong Jiang <jiangh99 at googlemail.com> wrote:
> 
> >  Hi,
> > The Hubbard U correction in LDA/GGA+U is to correct the failure of LDA/GGA
> > to describe localized d- or f-states.  Therefore LDA/GGA+U is able to give
> > an accurate band gap only if the band gap of your system happens to be of
> > d-d or f-f character, which, however, not the case for most systems (see,
> > e.g. H. Jiang, et al. *Phys. Rev. B* *82*, 045108 (2010)).<http://prb.aps.org/abstract/PRB/v82/i4/e045108>
> >
> > To obtain accurate band gaps, you can try
> > * modified Becke-Johnson, ( F. Tran and P. Blaha, Phys. Rev. Lett. *102*,
> > 226401 (2009)<http://link.aip.org/link/?&l_creator=getabs-normal1&l_dir=FWD&l_rel=CITES&from_key=JCPSA6000138000013134115000001&from_keyType=CVIPS&from_loc=AIP&to_j=PRLTAO&to_v=102&to_p=226401&to_loc=APS&to_url=http%3A%2F%2Flink.aps.org%2Fabstract%2FPRL%2Fv102%2Fe226401>;
> > H. Jiang (J. Chem. Phys. *134*, 134115(2013)<http://dx.doi.org/10.1063/1.4798706>)
> > * hybrid functionals, (F. Tran and P. Blaha, Phys. Rev. B 83, 235118
> > (2011) <http://dx.doi.org/10.1103/PhysRevB.83.235118>)
> > * GW (if your system is not very big).  (H. Jiang et al. Computer Phys.
> > Commun.,*184*, 348(2013) <http://dx.doi.org/10.1016/j.cpc.2012.09.018>).
> >
> >     Hong
> >
> >
> > 于 2013/7/20 14:03, Zaid 写道:
> >
> >    Dear Sir,
> >
> >  you are right. Experimental band gap for my compound is 2.0eV. The
> > maximum band gap on increasing U value for my calculation is 1.117eV at 10
> > eV. So, should I increase U value beyond 10eV ? Untill now from my
> > literature review, the value of U is less than 10 eV.
> >  Secondly, if suppose a compound has not been studied experimentally then
> > how we choose calue of U ?
> >
> >  Thank you
> >  Zaid
> >
> >
> > On Sat, Jul 20, 2013 at 1:48 PM, <tran at theochem.tuwien.ac.at> wrote:
> >
> >> Hi,
> >>
> >> I have never heard that the proper value of U is the one when the band gap
> >> starts to decrease. Usually the value of U is chosen such that the
> >> calculated properties (e.g., band gap or magnetic moment) agree with
> >> experiment.
> >>
> >> F. Tran
> >>
> >> On Sat, 20 Jul 2013, Zaid wrote:
> >>
> >> > Respected Users
> >> >
> >> >
> >> > I am applying GGA+U technique to considered compounds. In order to
> >> select
> >> > proper value of U for specific material, I am changing the value of U
> >> from
> >> > 2eV to 10eV with a step of 1eV. I was expecting that on increasing U
> >> value
> >> > in the range 2-10eV, a value of U will appear where the corresponding
> >> band
> >> > gap will start to decrease instead of increase. That value of U where
> >> the
> >> > band gap start to decrease instead of rising will be the proper U value
> >> in
> >> > the range 2-10eV.
> >> >
> >> > I my case the, every time I am getting increasing band gap values on
> >> > increasing U value from 2-10eV with a step of 1eV. Band gap does not
> >> > decrease in the range 2-10eV. Should I increase the range ? May anyone
> >> > suggest that whether my practice of finding U value is right ? If not
> >> how
> >> > can I find proper value of U for specific compound ?
> >> >
> >> > Thank you very much
> >> >
> >> > Best Regards
> >> > Zaid
> >> >
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