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

[pboulet]

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 <mailto: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 <mailto:nisidate at iwate-u.ac.jp>
> kazume.nishidate at gmail.com <mailto:kazume.nishidate at gmail.com>
> 
> 
> 2021年11月5日(金) 9:23 José Xavier <jxln_karate at yahoo.com.br <mailto: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 <mailto: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 <mailto:nisidate at iwate-u.ac.jp>
> kazume.nishidate at gmail.com <mailto:kazume.nishidate at gmail.com>
> 
> 
> 
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