Ar-C2H2 van der Waals Dimer: A Pathologically Flat Potential Energy Surface

Ab initio calculations at the second order Moller-Plesset perturbation theory (MP2) level with a large basis set which includes f functions on heavy atoms resolve some of the current spectroscopic ambiguities about the structure of the argon-acetylene van der Waals dimer. A value for the equilibrium separation of 3.8-3.9 angstrom is deduced and justified by invoking a more reasonable value of the van der Waals radius on carbon in acetylene and by comparison with other complexes. The calculations also reveal inadequacies in the several forms suggested hitherto for the intermolecular potential of this complex. In particular there are found to be four stationary points on the dimer surface, although in actuality the observed structure will be substantially vibrationally averaged due to a large-amplitude intermolecular bending motion. Nevertheless, it is demonstrated that these results are not obtained without some considerable effort and on such a shallow potential energy surface, numerical precision becomes an important factor. Comparison of results with different basis sets suggests that the reliability of earlier calculations on acetylene complexes should be questioned.