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

[Hong Jiang]

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


? 2013/7/20 14:03, Zaid ??:
> 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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