PULLING, PACKING AND STACKING - STRUCTURAL PROCLIVITIES OF SF6-(RARE GAS)N VANDERWAALS CLUSTERS

Molecular dynamics simulated annealing techniques based on the best available (anisotropic) pair potentials have been used to determine the structures and energies of the most important low-energy isomers of SF6-(Rg)n for Rg = Ar and Kr and n = 1-21. The patterns of growth and relative stability illustrate the competition between the "pulling" of the Rg-SF6 potential anisotropy and the tendency for the Rg solvent atoms to optimize their "packing", either through formation of a close-packed layer wrapped around the SF6 or through formation of "stacked" or nonwetting structures in which the solvent atoms are piled up on one face of the SF6 substrate molecule. These results suggest that SF6 will be fully solvated in large clusters formed with Ar atoms but will lie on the surface of those formed with Kr atoms, and examination of the relevant potentials suggests that it will also be fully solvated in clusters formed with Ne and He but not in those formed with Xe. These conclusions are in accord with existing experimental data for these systems but disagree with some predictions yielded by recent simulations of high-temperature heterogeneous clusters based on isotropic model potentials.