Path-integral Monte Carlo studies of para-hydrogen clusters

Path-integral Monte Carlo calculations have been used to study para-hydrogen clusters, (p-H2)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∼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-H2 clusters.