ABINITIO CLUSTER CALCULATIONS OF THE ELECTRON-DENSITY AND ELECTRIC-FIELD GRADIENT IN CORUNDUM

Ab initio calculations have been performed using mixed basis sets on two clusters carefully designed to model bulk corundum. Computational results on the clusters are compared with crystal Hartree-Fock results and with experiment for the deformation electron density and for the electric field gradient at the oxygen nucleus. Inclusion of polarization functions on central oxygen atoms in the clusters results in quantitative agreement with electron distributions derived from X-ray diffraction data. Although none of the ab initio cluster results for the oxygen electric field gradient tensor agree well with the experimental NMR values, agreement improves with increased basis set flexibility and better design of the cluster.