Separation between Fermi level and CBM/VBM

[Dhanjit]

Dear developers,
I have been trying to get values of energy separation between the VBM and CBM with the Fermi energy level. For reference I have attached a schematic of my problem [1.4 eV (CBM-Fermi), 2.0 eV (Fermi-VBM)]. The Fermi energy at QE DFT level is -0.2 eV. Which will remain same for GW as well, right? Now as the ypp shifts the VBM to zero, I get confused with how to get the energy separation at GW level.

[Daniele Varsano]

Dear Dhanjit,

the Fermi energy in a semiconductor at zero kelvin can be arbitrarily placed at the VBM, in the mid-gap or in other positions inside the gap. As this is arbitrary, in a semiconductor, by convention Yambo put the Fermi energy as zero at the VBM. In QE, the Fermi energy is computed by filling the lowest-energy Kohn-Sham states until the total number of electrons is reached and depending on if you used the occupations fixed or smearing it can be at the VBM or just slightly shifted.

If you are instead interested in its position wrt the vacuum level (i.e. the work function) you need to know where that level is which can be computed for non-periodic system but cannot be calculated from a bulk calculation only.

Best,
Daniele