CLUSTER STUDIES OF LA2CUO4CUO6

We present the results of ab initio calculations for a CuO6 cluster embedded in a background potential appropriate for La2CuO4. These calculations differ from prior work in that an improved embedding potential for the cluster is utilized which enforces orthogonality between the cluster electrons and the background point charges. This is accomplished through the use of effective core potentials to represent the point charges nearest the CuO6 cluster. The major qualitative change this brings about is a destabilization of the Cu4s level relative to the Cu3d orbital. The cluster results for n doping now give a 3d10 configuration for Cu as opposed to the 3d(9)4s configuration observed in earlier work. In addition, much more extensive treatments of electron correlation have been examined using the technique of configuration interaction (CI). Results for n-doped, undoped, and p-doped clusters are presented. The ligand --> metal charge transfer excitations, the crystal-field spectrum, and the photoemission spectrum are discussed. Special attention is given to the characterization of the wave function for the p-doped cluster. While the loss in electron density is almost entirely from the 02p(sigma) orbital, in agreement with experiment, it is shown that the wave function is closer to the independent-particle ''molecular orbital'' limit than the ''valence-bond'' local singlet, and that a description of the cluster ion ground states in terms of the occupation of a single ''effective'' Cu 3d orbital is possible.