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

[José Xavier]

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