[Wien] Confusion in Selecting proper value of Hubbard potential.
Peter Blaha
pblaha at theochem.tuwien.ac.at
Sun Jul 21 09:38:32 CEST 2013
People with experience "know" what U you should use (at least the range of
reasonable values). You can also "calculate" U (see our web-site), but the
calculated U values are often too large.
Since you do not tell us which system you have, we cannot help you. But for sure
bigger values of U than 8 eV are unphysical.
From what you tell us, I suppose you have 4f electrons, then after some value of U
the gap is 4f-5d and cannot be increase much by U anymore. If this is the case,
Hong Jiang's mBJ+U method should be good, as mBJ affects the 5d and U the 4f
states properly.
Am 20.07.2013 08:50, schrieb 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 <mailto: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 <mailto: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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Peter Blaha
Inst. Materials Chemistry, TU Vienna
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Tel: +43-1-5880115671
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email: pblaha at theochem.tuwien.ac.at
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