Melting and phase space transitions in small ionic clusters

We investigate the thermodynamical behavior of small neutral sodium fluoride 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 temperature and liquidlike at higher temperatures. However, unlike van der Waals clusters, the melting process appears to be a multistage phenomenon: 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 exponent lambda as a function of the total energy E. The slope of log lambda(E) strongly drops at the onset of first isomerization. The subsequent fluid transitions are not associated with mere changes in this slope. In view of this fact, we conclude that the degree of chaos in the dynamics of the system is not sensitive to the total volume of accessible phase space, once the lowest saddle points are allowed to be crossed.