<div dir="ltr"><div><div><div>Dear Sir H. Jiang<br><br></div>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.<br>
<br>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. <br>
Secondly, if suppose a compound has not been studied
experimentally then how we choose calue of U ?<br><br></div>Kindly guide.<br><br></div>Best regards<br>Zaid<br><div class="gmail_extra"><br><br><div class="gmail_quote">On Sat, Jul 20, 2013 at 2:31 PM, Hong Jiang <span dir="ltr"><<a href="mailto:jiangh99@googlemail.com" target="_blank">jiangh99@googlemail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div>Hi,<br>
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. <a href="http://prb.aps.org/abstract/PRB/v82/i4/e045108" target="_blank"><i>Phys. Rev. B</i> <b>82</b>,
045108 (2010)).</a><br>
<br>
To obtain accurate band gaps, you can try <br>
* modified Becke-Johnson, ( F. Tran and P. Blaha, <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">Phys.
Rev. Lett. <b>102</b>, 226401 (2009)</a>; H. Jiang (<a href="http://dx.doi.org/10.1063/1.4798706" target="_blank">J.
Chem. Phys. <b>134</b>, 134115(2013)</a> )<br>
* hybrid functionals, (F. Tran and P. Blaha, <a href="http://dx.doi.org/10.1103/PhysRevB.83.235118" target="_blank"><span>Phys.
Rev. B</span> <span style="font-weight:bold"><span style="font-weight:bold">83</span></span>, 235118 (2011)</a>)<br>
* GW (if your system is not very big). (H. Jiang et al. <a href="http://dx.doi.org/10.1016/j.cpc.2012.09.018" target="_blank">Computer Phys. Commun.,<b>184</b>,
348(2013)</a>).<br>
<br>
Hong<br>
<br>
<br>
于 2013/7/20 14:03, Zaid 写道:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>Dear Sir,<br>
</div>
<br>
</div>
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. <br>
</div>
Secondly, if suppose a compound has not been studied
experimentally then how we choose calue of U ?<br>
<br>
</div>
Thank you<br>
</div>
Zaid<br>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Sat, Jul 20, 2013 at 1:48 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">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 select<br>
> proper value of U for specific material, I am changing
the value of U from<br>
> 2eV to 10eV with a step of 1eV. I was expecting that on
increasing U value<br>
> in the range 2-10eV, a value of U will appear where the
corresponding band<br>
> gap will start to decrease instead of increase. That
value of U where the<br>
> band gap start to decrease instead of rising will be
the proper U value 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 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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