Embedded-cluster calculations for transition-metal impurities in

The embedded-cluster technique is used to simulate the local electronic structure of transition-metal impurities in . The description of the central defect cluster employs an ab initio SCF-MO approach. The quantum cluster consists of 21 ions. Outer crystal regions are modelled on the basis of a shell-model representation. In all cases defect-induced lattice relaxations have been consistently included. Our results, demonstrated for , concern optical transitions, Jahn - Teller effects and questions related to the stability of this defect. The computational level of our ab initio calculations corresponds to Hartree - Fock theory (HF) and the configuration interaction (CI). Additionally, Moller - Plesset perturbation theory and density functional theory have been applied to investigate charge-transfer transitions.