Correlation effects in ionic crystals: The cohesive energy of MgO.

High-level quantum-chemical calculations, using the coupled-cluster approach and extended one-particle basis sets, have been performed for (Mg)n(O )m clusters embedded in a Madelung potential. The results of these calculations are used for setting up an incremental expansion for the correlation energy of bulk MgO. This way, ∼ 96% of the experimental cohesive energy of the MgO crystal is recovered. It is shown that only ∼ 60% of the correlation contribution to the cohesive energy is of intra-ionic origin, the remaining part being caused by van der Waals-like inter-ionic excitations. accepted by Phys. Rev. B