The importance of high-order correlation effects for the CO–CO interaction potential

The CO–CO interaction energy is calculated for several geometries, both by the supermolecule MP4 and CCSD(T) methods, and by symmetry-adapted perturbation theory. Relatively large differences between the MP4 and CCSD(T) results are explained by means of a diagrammatic analysis of electron correlation effects, supported by quantitative calculations of the fifth-order contributions to the electrostatic interaction energy. It follows from this analysis that the calculation of an accurate intermolecular potential for CO is a particularly difficult problem: even the CCSD(T) method is not sufficiently reliable since it lacks important fifth-order correlation contributions.