band gap from BSE

[zchen]

Hi all:
I study molecule of Russian Doll. I calculated band gap using DFT and GW. I saw some paper which can extracted the band gap from BSE calcultion. I do not know how to get it from the BSE result. I can read the first peak of the spectrum. But it is far from the real band gap.

Zhifan Chen
Dept of Physics
Clark Atlanta University
USA

[myrta gruning]

Dear Zhifan Chen

What do you mean by "real band gap"?
From DFT+GW you get the fundamental band gap,
from BSE you get the optical band gap (then yes you can look at the first peak).
They correspond to are different physical processes and they have different values.

Regards,
Myrta

[Daniele Varsano]

Dear Zhifan
It is a matter of definition. It depends if you are interested in electronic gap, or optical gap!
The first is referred as the ionization energy minus the electron affinity, and it is accessible
by experiments using photoemission spectroscopy. The GW approximation is the way to
calculate it.

If you are interested in the optical gap, then you can look at the absorption. Note that
the first excitation can be a dark state (very small oscillator strengths), so no peaks appears
in the spectrum, it is anyway a solution of the BSE and you can look at it by using the ypp
utility.

So, it is important to know exactly what do you want to calculate and so, to which experiments
you want to compare. The optical and electronic are not equivalent, and in molecules they
strongly differs!!!

Cheers,

Daniele

[zchen]

Thanks all.