Negative heat capacity of sodium clusters
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.