Intermolecular potential of carbon dioxide dimer from symmetry-adapted perturbation theory

A four-dimensional intermolecular potential energy surface for the carbon d ioxide dimer has been computed using the many-body symmetry-adapted perturb ation theory (SAPT) and a large 5s3p2d1f basis set including bond functions . The SAPT level applied is approximately equivalent to the supermolecular many-body perturbation theory at the second-order level. An accurate fit to the computed data has been obtained in a form of an angular expansion inco rporating the asymptotic coefficients computed ab initio at the level consi stent with the applied SAPT theory. A simpler site-site fit has also been d eveloped to facilitate the use of the potential in molecular dynamics and M onte Carlo simulations. The quality of the new potential has been tested by computing the values of the second virial coefficient which agree very wel l with the experimental data over a wide range of temperatures. Our potenti al energy surface turns out to be substantially deeper than previous ab ini tio potentials. The minimum of -484 cm(-1) has been found for the slipped p arallel geometry at the intermolecular separation R = 3.54 Angstrom and a s addle point at -412 cm(-1) for the T-shaped configuration and R = 4.14 Angs trom. Three minima and two first-order saddle points have been located on t he pairwise-additive potential energy surface of the CO2 trimer. The nonpla nar structure of C-2 symmetry has been found to be 48.8 cm(-1) more stable than the cyclic planar form of C-3h symmetry, in disagreement with experime ntal observation. It is suggested that the relative stability of the two is omers cannot be reliably determined by pairwise-additive potential and incl usion of three-body forces is necessary for this purpose. (C) 1999 American Institute of Physics. [S0021-9606(99)30208-7].