Classical and quantum physics of hydrogen clusters

We present results of a comprehensive theoretical investigation of the low temperature (T) properties of clusters of para-hydrogen (p-H 2 ), both pristine as well as doped with isotopic impurities (i.e., ortho-deuterium, o-D 2 ). We study clusters comprising up to N = 40 molecules, by means of quantum simulations based on the continuous-space Worm algorithm. Pristine p-H 2 clusters are liquid-like and superfluid in the T → 0 limit. The superfluid signal is uniform throughout these clusters; it is underlain by long cycles of permutation of molecules. Clusters with more than 22 molecules display solid-like, essentially classical behavior at temperatures down to T ~ 1 K; some of them are seen to turn liquid-like at sufficiently low T (quantum melting).