[QE-users] Missing empty state in SCF calculation

[Kazume NISHIDATE]

Dear Lorenzo Sponza

> calculation of a gapped system (occupations = 'fixed'). If I include
> a large number of empty bands, these are the energies in Gamma:

You should always include some empty bands to get reliable band
energies for insulators and semiconductors.

> -4.8200-4.3169-0.9586-0.5478-0.5196-0.4977-0.0537 0.0095 with
> occupations 1.0000 1.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

So the band gap estimated at the gamma of your syste is Eg = 3.3583
eV. This Eg value does not solely guarantee that the system is in the
insulating state.

> Instead, if I include only one empty band the result is
> -4.8200-4.3169-0.5477 with occupations 1.0000 1.0000 0.0000 It's
> like in if the latter case I was missing the state at -0.9586 eV.

In this case the band gap estimated at the gamma is Eg = 3.7692 eV.


Is there no band-crossing over the BZ?
Did you check the band structure by plotting the energies
along some k-point path?

What kind of k-points mesh are you using for the SCF calculation?


>  but I was intrigued by this unexpected behaviour and I would like to understand what's happening.

Looks me like a numerical error induced by inappropriate setting 
(one empty band inclusion and/or complicated band structure).




西館数芽
Kazume NISHIDATE Ph.D

Department of Systems Innovation Engineering,
Graduate School of Science and Engineering, Iwate University
4-3-5 Ueda, Morioka, Iwate, 020-8551 JAPAN
Phone:+81-19-621-6391
kazume.nishidate at gmail.com, nisidate at iwate-u.ac.jp
https://sites.google.com/site/nisidatelab/