MELTING AND PHASE-SPACE TRANSITIONS IN SMALL IONIC CLUSTERS

We investigate the thermodynamical behavior of small neutral sodium fl uoride clusters using both Monte Carlo (MC) and molecular dynamics (MD ) simulations. The three clusters studied, (NaF)(n) with n = 6, 7, and 13, all display thermodynamical phases of a solidlike nature at low t emperature and liquidlike at higher temperatures. However, unlike van der Waals clusters, the melting process appears to be a multistage phe nomenon: The clusters undergo successive transitions toward more fully fluid states. We relate this behavior to the isomerization processes occurring in these systems and the topography of the potential energy surface. As one of our indicators, we calculate the largest Liapunov e xponent lambda as a function of the total energy E. The slope of log l ambda(E) strongly drops at the onset of first isomerization. The subse quent fluid transitions are not associated with mere changes in this s lope. In view of this fact, we conclude that the degree of chaos in th e dynamics of the system is not sensitive to the total volume of acces sible phase space, once the lowest saddle points are allowed to be cro ssed.