STRUCTURE, DYNAMICS, AND THERMODYNAMICS OF MODEL (H2O)8 AND (H2O)20 CLUSTERS

We present molecular dynamics simulations of (H2O)8 and (H2O)20, payin g particular attention to the possibility of solid-like/liquid-like co existence. Four differently parametrized rigid molecule potentials are examined for (H2O)8; Only the most promising is applied to (H2O)20. I n every case, we find evidence for time-scale coexistence in the stati stics of the short-time averaged temperature. In several cases, we als o observe loops in the microcanonical caloric curve [T(E)], indicating the formal existence of two stable states over a finite range of ener gy. Further evidence is provided by systematic quenching, both by comp arison with the dynamics and in terms of model density of states calcu lations of the microcanonical T(E), energy distribution function f (E) , Helmholtz free energy A ( T), and heat capacity C(v) ( T). We discus s two possible approaches to these thermodynamic functions from the di stribution of local energy minima and compare the results with those f or atomic clusters bound by the Lennard-Jones potential.