  We present results of a theoretical study of para-hydrogen and
ortho-deuterium clusters at low temperature (0.5 K < T < 3.5 K), based on Path
Integral Monte Carlo simulations. Clusters of size up to N=21 para-hydrogen
molecules are nearly entirely superfluid at T < 1 K. For 21 < N < 30, the
superfluid response displays strong variations with N, reflecting structural
changes that occur on adding or removing even a single molecule. Some clusters
in this size range display quantum melting, going from solid- to liquid-like as
T tends to 0. Melting is caused by quantum exchanges of molecules. The largest
para-hydrogen cluster for which a significant superfluid response is observed
comprises 27 molecules. Evidence of a finite superfluid response is presented
for ortho-deuterium clusters of size up to 14 molecules. Magic numbers are
observed, at which both types of clusters feature pronounced stability.
