Combining quasiparticle energy calculations with exact-exchange density-functional theory

We present a systematic {\it ab initio} study of the electronic structure for selected II-VI compounds and group III nitrides in the zinc-blende structure with special emphasis on analysing the role played by the semicore $d$-electrons. We show that applying density-functional theory (DFT) in the exact-exchange (EXX) approach [1] leads to an improved description of the $d$-electron hybridisation compared to the local-density approximation (LDA). Moreover we find that it is essential to use the newly developed EXX pseudopotentials [2] in order to treat core-valence exchange consistenly. In combination with quasiparticle energy calculations in the $GW$ approximation we achieve very good agreement with available photoemission data. Since the DFT energies and wavefunctions serve as input for the $GW$ calculation we conclude that for these materials EXX constitutes the better starting groundstate.\\ $[$1$]$ M. St\"adele {\it et al}, Phys.\ Rev.\ Lett. {\bf 79} 2089 (1997) \\ $[$2$]$ M. Moukara {\it et al}, J.\ Phys.: Condens.\ Matter {\bf 12} 6783 (2000)