Solid–liquid phase changes in simulated isoenergetic Ar13

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Abstract
Simulations by molecular dynamics of 13‐particle clusters of argon display distinct nonrigid, liquid‐like and near‐rigid, solid‐like ‘‘phases.’’ The simulations, conducted at constant total energy, display a low‐energy region in which only the solid‐like form appears, a high‐energy region in which only the liquid‐like form appears, and an intermediate band of energy—a ‘‘coexistence region’’— in which clusters exhibit both forms. The intervals of time spent in each phase in the two‐form coexistence region are long compared with the intervals required to establish equilibrium‐like properties distinctive of each form, such as mean square displacement and power spectrum, so that well‐defined phases can be said to exist. The fraction of time spent in each phase is a function of the energy. When a long simulation is separated into regions of solid‐like and liquid‐like behavior, the curve of the derived caloric equation of state is double valued in the two‐phase range of energy, forming two well‐defined, smooth branches. When, instead, the caloric curve is constructed from averages over all of a long run, its form is smooth and monotonic showing no trace of the ‘‘loop’’ that had been reported for earlier treatments with much shorter molecular dynamics runs, and which we could also reproduce with short runs.