qe-occupation

[sunxl]

Dear all，

1、I calculated a system using VASP, and the band gap was approximately 0.5 eV. When I used QE with

occupations = 'smearing'
smearing = 'gauss'
degauss = 0.01

after the conversion from the input form to that compatible with Yambo is completed, and after executing yambo, a prompt indicating a metallic system is obtained.

[02.05] Energies & Occupations
==============================

[X] === General ===
[X] Electronic Temperature : 0.258606E-1 300.100 [eV K]
[X] Bosonic Temperature : 0.258606E-1 300.100 [eV K]
[X] Finite Temperature mode : yes
[X] El. density : 0.35884E+23 [cm-3]
[X] Fermi Level : -3.570586 [eV]

[X] === Gaps and Widths ===
[X] Conduction Band Min : -3.570586 [eV]
[X] Valence Band Max : -3.570586 [eV]
[X] Filled Bands : 123
[X] Metallic Bands : 124 126
[X] Empty Bands : 127 600

[X] === Metallic Characters ===
[X] N of el / N of met el : 248.0000 2.0000
[X] Average metallic occ. : 0.166667

[WARNING] [X] Metallic system

However, when I used ypp to obtain the interpolated band structure, there was a gap. What is the reason for this? Will this warning have any impact on the subsequent calculation of the gw band?

2、When I set occupation = 'fixed', it seems that this problem won't occur. We know that for semiconductors and insulators, "fixed" should be used. But what about when the PBE band gap of a system is approximately 0.03 eV, how should the occupation be chosen? When the PBE band gap is zero and the energy bands have a slight overlap, that is, when it is a pseudo-metal, using the hse and gw methods will increase the band gap. In this case, how should the occupation be selected?


Best，
sunxl