LOCAL GEOMETRY AND RESONANT VIBRATIONS OF CU(-NAF - RESULTS OF AB-INITIO PERTURBED ION, CLUSTER-IN-THE-LATTICE CALCULATIONS INVOLVING CLUSTERS OF 179 IONS())

The ground state electronic structure and energy of a CuF92Na865- clus ter (Cu+ plus 12 shells of neighbors) embedded into a quantum lattice representing the NaF crystal are determined by using the ab initio per turbed ion (aiPI) method, with unrelaxed Coulomb-Hartree-Fock (uCHF) c orrelation energy corrections. Parallel calculations are performed on the NaF92Na865- cluster of the pure crystal in order to identify the c hanges induced by the impurity and to estimate the systematic errors i n our calculations. The geometry of the first four shells (32 ions) is allowed to relax by following symmetric breathing modes. An inwards r elaxation of - 0.12 angstrom is predicted for the nearest neighbors (n n) shell, but negligible relaxations are found for the outer shells. T he substitution of the Na+ ion by the Cu+ impurity is favored by - 1.0 3 eV. The Cu+ ion is found to occupy an on-center octahedral position. The 138 independent O(h) force constants corresponding to the vibrati on of the Cu+ and its first four shells of neighbors are then numerica lly computed from the aiPI energy using a Richardson iterated, finite- difference limit formula. These force constants give the vibrational m odes of the impurity center. Strong couplings are found among vibratio nal modes of adjacent shells. Vibration frequencies characteristic of the doped system are obtained at 206 cm-1 (1a1g), 108 cm-1 1t1u), and 173 cm-1 (1e(g)) determined mainly by the motions of the CuF6 octahedr on.