STRUCTURE AND DYNAMICS OF SIMULATED (SF6)N CLUSTERS IN THE SIZE RANGE N=7-55

Isolated (SF6)N clusters have been studied by molecular dynamics simulations in order to complete the temperature-size phase diagram for small clusters sizes (N less-than-or-equal-to 55). The transition between noncrystalline and crystalline ''bulk-like'' properties occurs in the size range between 13 and 19 molecules. This is two orders of magnitude below the range in which the same transition occurs in argon. Some evidence is provided of the existence of a triple point in the temperature-size phase diagram of SF6 at T approximately 60 K and N approximately 34 molecules. In very small clusters such as (SF6)7 and (SF6)13, the centers of masses of the octahedral sulfur hexafluoride molecules can accommodate to a fivefold symmetry with some distortions. Contrary to what had been observed in Lennard-Jones (LJ)13, no sign of a sharp ''transition'' with coexistence between rigid and nonrigid states was found here. This has been explained in terms of the existence of many quasidegenerate low energy states in the potential energy surface of small SF6 clusters.