<div dir="ltr">thanks a lot Sir for your guidance.<br></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sat, Jul 20, 2013 at 4:20 PM, <span dir="ltr"><<a href="mailto:tran@theochem.tuwien.ac.at" target="_blank">tran@theochem.tuwien.ac.at</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">You can try larger U to see what happens, but values above 10 or 12 eV<br>
start to be unphysical in solids. Also, the more the top of the valence<br>
band or bottom of conduction band has an important contribution not coming<br>
from the d or f electrons (e.g., oxygen-p), the less GGA+U will be<br>
efficient to adjust the band gap. GGA+U does not always solve problems in<br>
stronlgy correlated systems. As mentioned by Hong Jiang, you should maybe<br>
consider other methods.<br>
<br>
F. Tran<br>
<br>
On Sat, 20 Jul 2013, Zaid wrote:<br>
<br>
> Dear Sir H. Jiang<br>
><br>
> I implemented GGA+U to my considered compound, keeping in mind that GGA+U<br>
> is valuable for compounds having strongly correlated system (like in d and<br>
> f orbitals). But My problem is regarding the selection of U value.<br>
><br>
> Experimental band gap for my compound is 2.0eV. The maximum band gap on<br>
> increasing U value for my calculation is 1.117eV at 10 eV. So, should I<br>
> increase U value beyond 10eV ? Untill now from my literature review, the<br>
> value of U is less than 10 eV.<br>
> Secondly, if suppose a compound has not been studied experimentally then<br>
> how we choose calue of U ?<br>
><br>
> Kindly guide.<br>
><br>
> Best regards<br>
> Zaid<br>
><br>
><br>
> On Sat, Jul 20, 2013 at 2:31 PM, Hong Jiang <<a href="mailto:jiangh99@googlemail.com">jiangh99@googlemail.com</a>> wrote:<br>
><br>
> > Hi,<br>
> > The Hubbard U correction in LDA/GGA+U is to correct the failure of LDA/GGA<br>
> > to describe localized d- or f-states. Therefore LDA/GGA+U is able to give<br>
> > an accurate band gap only if the band gap of your system happens to be of<br>
> > d-d or f-f character, which, however, not the case for most systems (see,<br>
> > e.g. H. Jiang, et al. *Phys. Rev. B* *82*, 045108 (2010)).<<a href="http://prb.aps.org/abstract/PRB/v82/i4/e045108" target="_blank">http://prb.aps.org/abstract/PRB/v82/i4/e045108</a>><br>
> ><br>
> > To obtain accurate band gaps, you can try<br>
> > * modified Becke-Johnson, ( F. Tran and P. Blaha, Phys. Rev. Lett. *102*,<br>
> > 226401 (2009)<<a href="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" target="_blank">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</a>>;<br>
> > H. Jiang (J. Chem. Phys. *134*, 134115(2013)<<a href="http://dx.doi.org/10.1063/1.4798706" target="_blank">http://dx.doi.org/10.1063/1.4798706</a>>)<br>
> > * hybrid functionals, (F. Tran and P. Blaha, Phys. Rev. B 83, 235118<br>
> > (2011) <<a href="http://dx.doi.org/10.1103/PhysRevB.83.235118" target="_blank">http://dx.doi.org/10.1103/PhysRevB.83.235118</a>>)<br>
> > * GW (if your system is not very big). (H. Jiang et al. Computer Phys.<br>
> > Commun.,*184*, 348(2013) <<a href="http://dx.doi.org/10.1016/j.cpc.2012.09.018" target="_blank">http://dx.doi.org/10.1016/j.cpc.2012.09.018</a>>).<br>
> ><br>
> > Hong<br>
> ><br>
> ><br>
> > 于 2013/7/20 14:03, Zaid 写道:<br>
> ><br>
> > Dear Sir,<br>
> ><br>
> > you are right. Experimental band gap for my compound is 2.0eV. The<br>
> > maximum band gap on increasing U value for my calculation is 1.117eV at 10<br>
> > eV. So, should I increase U value beyond 10eV ? Untill now from my<br>
> > literature review, the value of U is less than 10 eV.<br>
> > Secondly, if suppose a compound has not been studied experimentally then<br>
> > how we choose calue of U ?<br>
> ><br>
> > Thank you<br>
> > Zaid<br>
> ><br>
> ><br>
> > On Sat, Jul 20, 2013 at 1:48 PM, <<a href="mailto:tran@theochem.tuwien.ac.at">tran@theochem.tuwien.ac.at</a>> wrote:<br>
> ><br>
> >> Hi,<br>
> >><br>
> >> I have never heard that the proper value of U is the one when the band gap<br>
> >> starts to decrease. Usually the value of U is chosen such that the<br>
> >> calculated properties (e.g., band gap or magnetic moment) agree with<br>
> >> experiment.<br>
> >><br>
> >> F. Tran<br>
> >><br>
> >> On Sat, 20 Jul 2013, Zaid wrote:<br>
> >><br>
> >> > Respected Users<br>
> >> ><br>
> >> ><br>
> >> > I am applying GGA+U technique to considered compounds. In order to<br>
> >> select<br>
> >> > proper value of U for specific material, I am changing the value of U<br>
> >> from<br>
> >> > 2eV to 10eV with a step of 1eV. I was expecting that on increasing U<br>
> >> value<br>
> >> > in the range 2-10eV, a value of U will appear where the corresponding<br>
> >> band<br>
> >> > gap will start to decrease instead of increase. That value of U where<br>
> >> the<br>
> >> > band gap start to decrease instead of rising will be the proper U value<br>
> >> in<br>
> >> > the range 2-10eV.<br>
> >> ><br>
> >> > I my case the, every time I am getting increasing band gap values on<br>
> >> > increasing U value from 2-10eV with a step of 1eV. Band gap does not<br>
> >> > decrease in the range 2-10eV. Should I increase the range ? May anyone<br>
> >> > suggest that whether my practice of finding U value is right ? If not<br>
> >> how<br>
> >> > can I find proper value of U for specific compound ?<br>
> >> ><br>
> >> > Thank you very much<br>
> >> ><br>
> >> > Best Regards<br>
> >> > Zaid<br>
> >> ><br>
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> ><br>
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<br></blockquote></div><br></div>