Correlations in ionic solids by means of ab initio quantum chemistry

Ab initio quantum-chemical methods are not necessarily restricted to molecules and have already been applied to calculate cohesive properties of semiconductors. We extend this method to ionic solids (MgO, CaO, NiO) and calculated cohesive energies and lattice constants. We obtain ∼ 80 % of the correlation contribution to the cohesive energy. Contributions due to the formation of ions are of the same order of magnitude for the cohesive energy as van der Waals-like interactions between the ions. Including correlations, the calculated lattice deviate by less than 1 % from the experimental values. Two main effects arising from correlations are found: the van der Waals-like interaction between the ions reduces the lattice constant whereas intra-atomic correlation of the oxygen ion enforces a larger lattice constant due to the lower level spacing and therefore increasing importance of correlations at a larger lattice constant. First and second ionization potential values of magnesium, calcium, and nickel were calculated. (authors)