ABINITIO CALCULATION OF THE LOCAL GEOMETRY OF CU+NAF AND CU+NACL

A new description of the nature and scope of the impurity centers Cu:NaF and Cu:NaCl emerges when the local equilibrium geometry and wave function are obtained from cluster-in-the-lattice calculations involving clusters from 7 to 33 ions. The numerical results reveal the importance of simulating the defects by means of clusters having, at least, a boundary shell of fixed ions whose wave functions can follow the geometrical changes of the cluster inner shells. Inward relaxations of 0.089 angstrom for Cu:NaF and 0.085 angstrom for Cu:NaCl are deduced from the best calculations, in agreement with recent measurements on the last system.