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 (${\mathrm{Mg}}^{2+}$${)}_{\mathit{n}}$(${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$${)}_{\mathit{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 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.