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 (Mg2+)(n)(O2-)(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 bulb MgO. This way approximately 96% of the experimental cohesive energy of the MgO crystal is recovered. It is shown that only about 60% of the correlation contribution to the cohesive energy is of intraionic origin, the remaining part being caused by van der Waals-like interionic excitations.