p-H2 vs o-D2 clusters: From liquid-like to glass-like behavior

Motivated by fascinating structural properties, as well as emerging numerical challenges, para-hydrogen clusters have been explored in numerous publications in the past. Despite the enormous computational resources used by a number of groups, involving various methods, no consensus on the clusters' energetic and structural properties has been established. Most studies reported strong size dependencies, e.g., "magic number" clusters, while strongly disagreeing with each other quantitatively. Only a few studies claimed the lack of size-sensitivity. That is, hardly more than a couple of reports could be considered numerically converged and/or physically meaningful. Unlike most of the previous studies, we focus on a small size range of Lennard-Jones LJ34-39 clusters, for which, using Diffusion Monte Carlo (DMC), we attempt to capture the true behavior of the systems accurately. Not only do we demonstrate that the (p-H2)N clusters for the chosen sizes have the ground state wavefunctions strongly delocalized over thousands of structurally identical isomers, but we also vary the quantum delocalization parameter Λ in the effective range between the hydrogen (ΛH2∼0.28) and deuterium (ΛD2∼0.20) regimes. We show that for the o-D2 clusters, the ground state wavefunctions are strongly localized, yet they are still disordered. In this regime, the system dynamics associated with the DMC method becomes non-ergodic, e.g., the random walkers get trapped in the potential energy minima where they have been initialized. (It is ergodic for hydrogen clusters.) Consequently, we suggest that the structural change induced by decreasing the quantum parameter Λ from p-H2 to o-D2 has the character of a "liquid-glass" transition.