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

[Zaid]

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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