New technique in the calculation of defects in solids by molecular methods: Pure and Cu-doped ZnS

We present a modification in the molecular-cluster methods for the calculation of defects in solids. The modification introduces a common energy reference for the perfect-cluster and defect-cluster calculations. Thus it is possible to compare the eigenvalues coming from the two calculations. Once this possibility is open, we discuss the applicability of the Koopmans theorem or of the transition state in the interpretation of the calculated results. We use one or the other, depending on the bandwidth, the self-energy, and the cluster size. The whole procedure is applied to pure ZnS and ZnS:Cu with very satisfactory results.