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
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--
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
_______________________________________________
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 list [email protected]
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