[QE-users] occupation in biological molecules and band gap

[Kazume NISHIDATE]

> So, a new calculation of the bandgap was performed with HSE to obtain
> the difference between Egap HSE and Egap PBE.

Ok, they used the 'scissor' operator to correct the PBE dielectric constant.

> If I remember the explanation, this value is added in the optical
> results to push all the results obtained with PBE toward the value
> that could be found using the HSE functional.

And they concluded that the correction was small despite the
underestimation tendency of the E_g_PBE (left column, p4).

> What do you suggest to me? What value should I use instead of the
> 2x2x2?

The 2x2x2 k-mesh must be used for the 'scf' calculation.  And then you
should try to evaluate the band structure along the specific k-point
path with the 'bands' (non-scf) calculation. This is the procedure they
used to evaluate the bands.

Have you ever done a band structure calculation of simple
semiconductor crystal? You should once try to draw a band structure
of Si (diamond structure) so that you can understand the procedure.



best regards
kazume NISHIDATE
敬具 西館数芽

nisidate at iwate-u.ac.jp
kazume.nishidate at gmail.com


2021年11月5日(金) 9:23 José Xavier <jxln_karate at yahoo.com.br>:

> Dear,
> About the 'scissor', it was used because a reviewer asked to also perform
> the calculations with a hybrid functional. So, a new calculation of the
> bandgap was performed with HSE to obtain the difference between Egap HSE
> and Egap PBE. If I remember the explanation, this value is added in the
> optical results to push all the results obtained with PBE toward the value
> that could be found using the HSE functional. So, 3.8 eV is the bandgap
> with PBE, while the HSE result is 4.7 eV.
>
> >In other wards, you should correctly plot the band structure along
> typical k-points to
> find the 'reasonable' band gaps.
>
> What do you suggest to me? What value should I use instead of the 2x2x2?
>
>
> Best Wishes
> José Xavier
>
> Em quinta-feira, 4 de novembro de 2021 07:27:48 BRT, Kazume NISHIDATE <
> nisidate at iwate-u.ac.jp> escreveu:
>
>
>
>
>
> Dear Stefano:
> > For isolated molecules, smearing helps dumping the convergence
> > oscillations arising from HOMO-LUMO level crossings.
>
> I did not know it.
> Thank you for the information.
>
>
> Dear José Xavier,
> > The band gap for Dopamine was 3.8 eV, and the other papers published
> >  by them showed values between 3-5 eV. That is why I have asked
>
> According to the paper you mentioned, they utlized the 'scissor'
> correction on the band gap,
>
>  E_g_scissor = E_g_HSE - E_g_PBE
>
> This should be the possible reason of the discrepancies.
>
> It is somewhat empirical way, but surprisingly, it looks me work for some
> transition oxide materials, according to their paper. As far as I know,
> the 'scissor' method is not well recognized in the DFT community.
> #This is the first time I have ever heard of.
>
> And there is one other point. The valence band maximum (VBM), which
> corresponds to the HOMO, is located at the Y point in the BZ (see
> Fig.2 of the paper).  To estimate correctly the gap from the DOS
> analysis, it must include the contribution from the Y point.  The
> 2x2x2 mesh you mentioned is apparently insufficient.  In other wards,
> you should correctly plot the band structure along typical k-points to
> find the 'reasonable' band gaps.
>
>
>
> best regards
> kazume NISHIDATE
> 敬具 西館数芽
>
> nisidate at iwate-u.ac.jp
> kazume.nishidate at gmail.com
>
>
>
>
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