[QE-users] K-points, smearing and hybrid functionals in metals.

[Lorenzo Paulatto]

It could be just one case of Hartree Fock giving bad results for 
metallic systems, i.e. Phys. Rev. B 20, 1504 (1979), in particular the 
steep bands imply a vanishing DOS at the Fermi energy. But I'm not at 
all an expert of this kind of applications...

cheers

On 21/09/2022 16:54, Lorenzo Sponza wrote:
>
> Hello everybody.
>
> I'm having some troubles in calculating the band structure of bulk 
> Iridium with HSE06. In order to do that, I'm using the 'fake k-points' 
> method, so I extract the band structure from a SCF calculation using 
> an home-made post-processing code which is attached to this email. I 
> run each simulation in a different folder to be sure not to overwrite 
> important files. I'm using the marzari-vanderbilt smearing method with 
> degauss values ranging from 0.011 to 1.0 and regular k-point grids 
> ranging from 2x2x2 to 12x12x12 while the fake k-points list counts 26 
> k-points in all cases. You can find an example of input file attached 
> to the email. In parallel I have done also some PBE calculations with 
> the standard procedure SCF+BANDS.
>
> In the case of the PBE calculation, I don't observe differences in the 
> band dispersion depending on the degauss value, but the Fermi level 
> moves up for higher values of degauss and the k-point convergence is 
> faster.
>
> What I observe in the case of HSE06 is that the dispersion itself does 
> change depending on the degauss value. Please see the attached graphs. 
> In particular, for quite "standard" values of the degauss (0.02), the 
> band structure presents weird jumps crossing the Fermi level, whereas 
> for high values ( degauss = 0.8 ) the dispersion is smoother across 
> Fermi, converges very quickly with the regular k-point grid and looks 
> somewhat similar to the PBE one. I understand it as a consequence of 
> the fact that hybrids have a certain amount of EXX which act only on 
> occupied states. Since the occupation is not abrupt in metals and is 
> modified by the smearing, then the potential changes in a continuous 
> way as a function of degauss.
>
>
> Hence my questions:
>
> - In PBE calculations, how can I know what is the right value of the 
> degauss parameter? I saw answers in the forum as if it is a compromise 
> between k-points and smearing amplitude and ideally an infinitely 
> dense grid would give the exact result with no smearing. However this 
> answer does not help me in understanding where is the right Fermi 
> energy because a finer grid does not solve the issue.
>
> - Are hybrid functionals a safe choice for the calculation of the 
> electronic structure and band alignment in metals?
>
> - Is there a rule (rigorous or rule of thumb) to set the degauss value 
> in metals when using hybrid potentials?
>
> - Am I completely wrong, and I'm just making some mistake at the level 
> of scf or of the band extraction? (I have no experience on metals and 
> little on hybrids)
>
> Thanks a lot for your help !
>
>
> -- 
> Dr. Lorenzo Sponza
> Chargé de Recherche au CNRS
> Laboratoire d'étude de microstructures (LEM), CNRS-ONERA
> 29 Avenue de la division Leclerc, 92322 Châtillon
> Tel: +33146734464
>
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing listusers at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
phone: +33 (0)1 442 79822 / skype: paulatz
http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20220922/29e2bb42/attachment.html>