Dear all,

I'm calculating the absorption spectrum of CuCl with Yambo.

The band gap of CuCl observed in experiments is about 3.4 eV, but PBE only gives Eg~0.6 eV, this is of course entirely normal for DFT. To build the BSE Hamiltonian, I used scissor operator ΔE=2.8 eV to mimic the "real" quasiparticle transitions, and calculated the static screened Coulomb potential W from PBE band structure. However, after diagonalizing BSE Hamiltonian, surprisingly I find the exciton binding energy to be 130 meV, very similar to its experimental value 190 meV.

Although it's a good news to obtain comparable result against experiments, I'm still puzzled that why we can get such "good" binding energy with the screened potential calculated from such "bad" electronic structure. I.e., the PBE band gap is one order of magnitude smaller than the experimental value, but the exciton binding energy obtained from PBE screened potential is rather similar with the experimental value.

So, I want to know whether the such kind of results are reasonable. Also, how can we understand the good performance of PBE in calculating binding energy, or is it just a coincidence in CuCl ?

Thank you,