Melting and freezing in isothermal Ar13 clusters

Abstract

Microcanonical simulations have shown that Ar1 3 clusters have sharp but unequal melting and freezing energies. Between these energies, a hot solid‐like form and a cooler, liquid‐like form coexist in dynamic equilibrium. Monte Carlo and isothermal molecular dynamics simulations confirm that this coexistence behavior persists under canonical conditions as well. Many properties demonstrate the solid and liquid character of the two coexisting ‘‘phases.’’ One previous result seemed to contradict this: Quirke and Sheng evaluated nearest neighbor angular distribution function P(θ); its nonzero value for θ=π/2 at 33 K was interpreted as that of a hot solid in a ‘‘premelting expansion.’’ Actually, that result is the average of a bimodal distribution, one mode for the solid and the other for the liquid. The average shifts smoothly with T, and each form’s P(θ) changes slightly with temperature. The solid has tiny nonzero probability for π/2. The liquid has a minimum probability there, but far above zero. Mean‐square displacements and power spectra calculated at 33 K from the Nosé constant temperature molecular dynamics method exhibit properties which are clearly distinguishable and identifiable with two distinct phases, as they are under isoergic conditions. Hence our results can be added to the evidence supporting the picture for finite systems of two phases coexisting over a finite temperature and energy range.