Quasiparticle band structure of ZnS and ZnSe

We calculate the quasiparticle band structure of ZnS and ZnSe using the plane-wave pseudopotential method and the GW approximation for the self-energy. The Zn semicore 3d states are treated as valence states. A systematic study of the role of various approximations is presented, including the local density approximation (LDA) and generalized gradient approximation (GGA) for the ground-state properties, the role of self-consistency both in the dielectric function and the self-energy Sigma, the effect of off-diagonal matrix elements of (Sigma-V-xc) in the Kohn-Sham orbital basis, and the plasmon-pole approximation. This study demonstrates that the LDA and GGA give similar results, and that self-consistency in updating the quasiparticle energies improves the accuracy of the band gap as well as the energies of the semicore 3d states. The calculated quasiparticle band gaps of ZnS and ZnSe agree well with experimental values when the GW approximation is used. There is some discrepancy for the calculated quasiparticle energies of the semicore states and some possible reasons are discussed.