A model study of aggregation of acetylene molecules

An interaction potential previously developed for the acetylene dimer was applied to small and large clusters of acetylene molecules. With the inclusion of the effects of vibrational dynamics via quantum Monte Carlo techniques, the calculation of rotational constants of the trimer and tetramer gave a very good assessment for the use of the interaction potential with more than two interacting molecules. Calculations performed on larger clusters reveal trends in the energetics of aggregation and in the preferred arrangement of acetylenes. The pentamer is found to have essentially the structure of the cyclic tetramer with the additional molecule above and in the center of the ring. By continuing to very large clusters of up to 391 interacting molecules, it is clear in what ways the features of the crystal structures of acetylene appear in small clusters. Estimates of the lattice energy were accomplished through a limited extrapolation of the cluster results. The static-structure interaction energies of the cubic and orthorhombic crystals were found to be quite close, but with the high-temperature form being the lower by about 12% of the interaction energy. A number of refinements were evaluated to provide understanding of the delicate balancing of competing effects that are responsible for the existence of the two crystal farms of solid acetylene.