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

[José Xavier]

Dear, 

Thank you for your advice.

Yes. I followed some tutorials that I found on the internet, most of them used Silicon and Graphene. 

Best wishes.
José Xavier






Em quinta-feira, 4 de novembro de 2021 22:57:55 BRT, Kazume NISHIDATE <nisidate at iwate-u.ac.jp> escreveu: 







> 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
> 
> 
> 
>