Quantum Melting and Isotope Effects from Diffusion Monte Carlo Studies of p-H2 Clusters.We present a rigorous characterization of the ground state structures of p-H2 clusters and their isotopologues using diffusion Monte Carlo combined with the inherent structures analysis.  For the N = 19 cluster we explore the effect of "quantum melting" by quantifying the contributions of local minima to the ground state as a function of continuously varying particle mass.  Doubling the cluster size leads to an enormous increase of its complexity: the ground state of (p-H2)38 is highly delocalized over a large number of minima representing all the funnels of the potential energy surface.  The ground state of (o-D2)38 is also delocalized, but over a smaller subset of minima, which exclusively belong to the same disordered motif.
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