  Heat capacities of Na_N, N = 13, 20, 55, 135, 142, and 147, clusters have
been investigated using a many-body Gupta potential and microcanonical
molecular dynamics simulations. Negative heat capacities around the cluster
melting-like transition have been obtained for N = 135, 142, and 147, but the
smaller clusters (N = 13, 20, and 55) do not show this peculiarity. By
performing a survey of the cluster potential energy landscape (PEL), it is
found that the width of the distribution function of the kinetic energy and the
spread of the distribution of potential energy minima (isomers), are useful
features to determine the different behavior of the heat capacity as a function
of the cluster size. The effect of the range of the interatomic forces is
studied by comparing the heat capacities of the Na_55 and Cd_55 clusters. It is
shown that by decreasing the range of the many-body interaction, the
distribution of isomers characterizing the PEL is modified appropriately to
generate a negative heat capacity in the Cd_55 cluster.
