no bound exciton

[xixi]

Dear all;
I always met such cases, the GW gap is smaller than the BS energy, that means the binding energy is minus, in some case, I strength the K sampling and the problem solved, but in many other cases, it didn't work, well, could anyone give some advice on this issue? how to determine a system whether can bind exciton or not? Thanks

Xi Zhu
Nanyang University
Singapore,639798

[andrea marini]

Dear all;
I always met such cases, the GW gap is smaller than the BS energy, that means the binding energy is minus, in some case, I strength the K sampling and the problem solved, but in many other cases, it didn't work, well, could anyone give some advice on this issue? how to determine a system whether can bind exciton or not? Thanks

The fact that the GW gap is smaller that the lowest BS energy is not a problem, in general. There is no strict, exact rule to determine whether your system is gonna have a bound exciton. So, please, give us more details about your system: is it a metal, a semicondutor or an insulator. How large is the DFT/GW gap ? Home many atoms, which type of atoms .... and so on.

From a very very general point of view you could say

1. larger the gap, larger the probability to have a bound exciton
2. larger the dis-homogeneity, larger the probability to have a bound exciton
3. metal = no bound excitons
4. ...

Cheers

Andrea