PATH-INTEGRAL MONTE-CARLO STUDIES OF PARA-HYDROGEN CLUSTERS

Path-integral Monte Carlo calculations have been used to study para-hydrogen clusters, (p-H-2)N, with N = 13, 19, 33, and 34. Particular attention has been given to the-low temperature structures and their evolution with increasing temperature. The structures of these quantum clusters have a clear relationship to their classical counterparts. In particular, the core of the N = 19, 33, and 34 clusters is bipyramidal, in constrast to that of the N = 13 cluster, which is icosahedral. At T approximately 2 K, these clusters exhibit a large propensity for exchanging particles, even without explicitly including exchange permutations (Bose statistics). This behavior has its origin in the large zero-point motion of the very weakly bound small p-H-2 clusters.