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 : [email protected] <mailto:[email protected]> > Le 5 nov. 2021 à 02:57, Kazume NISHIDATE <[email protected]> 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 > > [email protected] <mailto:[email protected]> > [email protected] <mailto:[email protected]> > > > 2021年11月5日(金) 9:23 José Xavier <[email protected] > <mailto:[email protected]>>: > 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 > <[email protected] <mailto:[email protected]>> 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 > > [email protected] <mailto:[email protected]> > [email protected] <mailto:[email protected]> > > > > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu) > users mailing list [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users
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