[QE-users] Missing empty state in SCF calculation

[Lorenzo Sponza]

Dear all,

I encounter an issue I did not expect and I would like to know why it 
happens and how I am suppose to handle it. I'm running an SCF 
calculation of a gapped system (occupations = 'fixed'). If I include a 
large number of empty bands, these are the energies in Gamma:

     -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

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. 
How it happens ?

I'm rising this issue because I'm checking the gapwidth of a series of 
similar systems. I actually thought that it was safe to run simple SCF 
calculations with 1 empty state, but I realise that results are possibly 
wrong. Shall I mandatorily rely on NSCF calculations or I'm making a 
mistake somewhere ?

Best regards

-- 
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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