CORRELATIONS IN IONIC SOLIDS BY MEANS OF AB-INITIO QUANTUM-CHEMISTRY

Ab initio quantum-chemical methods are not necessarily restricted to m olecules and have already been applied to calculate cohesive propertie s of semiconductors. We extended this method to ionic solids (MgO, CaO , NiO) and calculated cohesive energies and lattice constants. We obta in approximate to 80 % of the correlation contribution to the cohesive energy. Contributions due to the formation of ions are of the same or der of magnitude for the cohesive energy as van der Waals-like interac tions between the ions. Including correlations, the calculated lattice constants deviate by less than 1 % from the experimental values. Two main effects arising from correlations are found: the van der Waals-Li ke interaction between the ions reduces the lattice constant whereas i ntra-atomic correlation of the oxygen ion enforces a larger lattice co nstant due to the lower level spacing and therefore increasing importa nce of correlations at a larger lattice constant.