#3
Post
by mkhorton » Thu Jul 29, 2021 11:07 pm
Hi Marie-Therese,
Thank you very much for your reply!
This is partly a problem of our own making. Essentially, to determine the band gap of a material, we typically assume that for a semiconductor that the Fermi level reported by VASP will be somewhere in the gap, and that states below this level are occupied and states above this level are unoccupied. This allows us to reduce storage requirements when we're storing 100,000s of band structures in our database since we don't have to store the occupancies explicitly.
Historically, this approach seems to have worked well. However, after running calculations with VASP 6 we started to notice that the band gap would change radically for existing materials in a small number of cases. It appears to be that the Fermi level lies within a band even for semiconductors where it clearly shouldn't. Examining these calculations with reference to the actual occupancies reported in the EIGENVAL file allows us to determine the correct band gap.
When comparing band gaps determined with reference to the Fermi level, and band gaps determined by looking at occupancies explicitly, these values largely agree for VASP v5.x, but for VASP 6.1.0 we see a large discrepancy. This is really only something we see in aggregate. It only affects a small % of calculations, perhaps hundreds out of our whole dataset.
We're still investigating this and a colleague is looking to find a small, reproducible example, so we don't have the full story here yet. I'll keep you updated if we learn more.
Best regards,
Matthew
