Structure, dynamics, and thermodynamics of model (H2O)8 and (H2O)20 clusters

Abstract

We present molecular dynamics simulations of (H2O)8 and (H2O)20, paying particular attention to the possibility of solid-like/liquid-like coexistence. Four differently parametrized rigid molecule potentials are examined for (H2O)8; only the most promising is applied to (H2O)20. In every case, we find evidence for time-scale coexistence in the statistics of the short-time averaged temperature. In several cases, we also observe loops in the microcanonical caloric curve [T(E)], indicating the formal existence of two stable states over a finite range of energy. Further evidence is provided by systematic quenching, both by comparison with the dynamics and in terms of model density of states calculations of the microcanonical T(E), energy distribution function f(E), Helmholtz free energy A(T), and heat capacity Cv(T). We discuss two possible approaches to these thermodynamic functions from the distribution of local energy minima and compare the results with those for atomic clusters bound by the Lennard-Jones potential.