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

[Kazume NISHIDATE]

Dear Pascal Boulet:

Thank you for the information.😃
It's all about the energy gap.
And still it is a problem we are tackling.


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) 18:57 pboulet <pascal.boulet at univ-amu.fr>:

> Dear all,
>
> Regarding the ‘scissor’ operator, when I was a PhD student working on
> molecular systems with programs using Slater types of orbitals (ADF), I
> sometimes had to use the so-called ‘level-shift’ trick to facilitate the
> SCF convergence (it avoids the swapping of occupied/unoccupied orbitals
> from one step to the next when the gap is small). If I am right, correct me
> if I am wrong, this level-shift trick ressembles a lot the ‘scissor’
> operator.
>
> Best,
> Pascal
>
>
> Pascal Boulet
> —
> *Professor in computational materials chemistry - DEPARTMENT OF CHEMISTRY*
> University of Aix-Marseille - Avenue Escadrille Normandie Niemen -
> F-13013 Marseille - FRANCE
> Tél: +33(0)4 13 55 18 10 - Fax : +33(0)4 13 55 18 50
> Email : pascal.boulet at univ-amu.fr
>
>
>
>
>
>
>
>
>
>
> Le 5 nov. 2021 à 02:57, Kazume NISHIDATE <nisidate at iwate-u.ac.jp> a écrit
> :
>
>
>
> > 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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