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

Sat Jul 20 12:18:47 CEST 2013

thanks a lot Sir for your guidance.

On Sat, Jul 20, 2013 at 4:20 PM, <tran at theochem.tuwien.ac.at> wrote:

> 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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> > >
> > >
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