Electronic band gap vs optical band gap

[burkzdemir]

Dear Yambo Developers,

I am running GW+BSE calculations and obtaining dielectric functions. However, when I look at the absorbance, I see that the optical band gap (1 eV) is smaller than the electronic band gap (2 eV). How is this possible?

Best,

[Daniele Varsano]

Dear Burak,
this is always the case when you are in the presence of bound excitons. The difference between the electronic bandgap and the optical gap is indeed the binding energy of the exciton due to the electron-hole attraction, i.e. the energy needed to separate the electron-hole pair in two independent electrons and holes.

Best,
Daniele