The importance of high-order correlation effects for the CO-CO interactionpotential

The CO-CO interaction energy is calculated for several geometries, both by the supermolecule MP4 and CCSD(T) methods, and by symmetry-adapted perturba tion theory. Relatively large differences between the MP4 and CCSD(T) resul ts are explained by means of a diagrammatic analysis of electron correlatio n effects, supported by quantitative calculations of the fifth-order contri butions to the electrostatic interaction energy. It follows from this analy sis that the calculation of an accurate intermolecular potential for CO is a particularly difficult problem: even the CCSD(T) method is not sufficient ly reliable since it lacks important fifth-order correlation contributions. (C) 1999 Elsevier Science B.V. All rights reserved.