3 December 1999 Ž . Chemical Physics Letters 314 1999 326–332 www.elsevier.nlrlocatercplett The importance of high-order correlation effects for the CO–CO interaction potential Michal Rode a , Joanna Sadlej a , Robert Moszynski a,b,1 , Paul E.S. Wormer b , Ad van der Avoird b, ) a Department of Chemistry, UniÕersity of Warsaw, Pasteura 1, 02-093 Warsaw, Poland b Institute of Theoretical Chemistry, NSR Center, UniÕersity of Nijmegen, ToernooiÕeld 1, 6525 ED Nijmegen, The Netherlands Received 2 August 1999; in final form 7 October 1999 Abstract Ž. The CO–CO interaction energy is calculated for several geometries, both by the supermolecule MP4 and CCSD T Ž. 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 Ž. 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. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction The intermolecular forces which determine the properties of molecular gases, liquids, and solids can be investigated in great detail by the study of Van wx der Waals complexes 1 . The theoretical and labora- tory studies of the high-resolution spectra of such complexes are also of importance for executing searches for them in atmospheres and interstellar clouds. Carbon monoxide is one of the constituents of the Earth’s atmosphere and of interstellar clouds, and the CO dimer was the subject of various theoret- ical and experimental studies. The first spectroscopic ) Corresponding author. Fax: q31-24-3653041; e-mail: avda@theochem.kun.nl 1 Also at Laboratoire de Chimie Theorique, UMR 7551 ´ CNRSrULP, Institut Le Bel, Universite Louis Pasteur, 4 rue ´ Blaise Pascal, F-67008 Strasbourg Cedex, France. observation of this dimer was reported by Vanden wx Bout 2 in 1979 in a molecular beam radiospectro- wx scopic experiment. Later, Havenith et al. 3 and wx Brookes and McKellar 4 reported more detailed Ž . spectroscopic studies of CO in the mid-infrared. 2 The CO dimer was also studied by other experimen- wx tal techniques. Schramm and collaborators 5 mea- sured the second virial coefficients of pure CO gas over a wide range of temperatures. Several experi- mental studies reported measurements of various wx transport properties of the CO gas 6 and of the lattice energy and phonon frequencies of the CO wx molecular crystal 7 . Surprisingly, only two ab initio potential energy w x surfaces are available for this system 8,9 . Van der wx Pol et al. 8 computed the CO–CO potential as the sum of the exchange–repulsion term approximated by the Heitler–London energy, and damped multi- pole expansions for the electrostatic, induction, and 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01168-9