Negative heat capacity of sodium clusters

Heat capacities of NaN, $N=13,$ 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 meltinglike 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 Na₅₅ and Cd₅₅ 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 Cd₅₅ cluster.