The energy loss of large molecular-hydrogen clusters incident on alumi
num and amorphous carbon targets is analyzed as a function of the clus
ter size and velocity, using a dielectric formalism to describe the el
ectronic interactions between the projectile and the target. The depen
dence of the energy loss with the cluster size predicts a marked diffe
rent behavior for aluminum or amorphous carbon. The energy loss of H-2
clusters in aluminum indicates that for each cluster velocity there i
s a ''resonant'' cluster size for which the intermolecular contributio
n to the energy loss is maximum. On the other hand, the intermolecular
contribution to the energy loss of H-2 clusters in amorphous carbon s
aturates with the cluster size. The origin of this behavior lies in th
e low-energy plasmon present in the energy loss function of amorphous
carbon. We also discuss the consequences of a proper description of th
e dielectric properties of the target.