AN INVESTIGATION OF THE STRUCTURE OF WEAKLY BOUND (OCS)(2)

Ab initio calculations confirm that there is a stable structure of the van der Waals dimer of OCS having C2h symmetry, with the sulphur atoms occupying the inner positions in the complex, in accordance with a recent experimental observation. Second-order Moller-Plesset perturbation theory (MP2) calculations with a TZ2P basis set find that this structure is the global minimum. Three other stationary points, predicted by the electrostatic model, have been located. Two of these have the monomer axes parallel, but staggered, with respect to one another; in the other the monomers are collinear. A comparison of self-consistent field (SCF) and MP2 results demonstrates that although the observed form is an electrostatically favoured structure, dispersion interactions are crucial in determining its energy relative to the other isomers. What becomes abundantly clear is that basis sets of the size of double-zeta with a single shell of polarization functions are inadequate to describe species such as (OCS)2.