  We introduce a new criterion--based on multipole dynamical correlations
calculated within Reptation Quantum Monte Carlo--to discriminate between a
melting vs. freezing behavior in quantum clusters. This criterion is applied to
small clusters of para-hydrogen molecules (both pristine and doped with a CO
cromophore), for cluster sizes around 12 molecules. This is a magic size at
which para-hydrogen clusters display an icosahedral structure and a large
stability. In spite of the similar geometric structure of CO@(pH2)_12 and
(pH2)_13, the first system has a rigid, crystalline, behavior, while the second
behaves more like a superfluid (or, possibly, a supersolid).
