QUANTUM CLUSTER EQUILIBRIUM-THEORY OF LIQUIDS - ILLUSTRATIVE APPLICATION TO WATER

The Quantum Cluster Equilibrium (QCE) theory, developed in a previous paper, is illustrated with a simple 7-cluster RHF/3-21G model for liqu id water. We first describe a simplified QCE(0) model that corresponds to an ideal gas cluster mixture (neglect of all cluster-cluster inter actions), but is nevertheless able to exhibit many characteristic feat ures of a true gas/liquid phase transition (macro-clustering, volume c ollapse, specific heat increase, Clausius-Clapeyron pressure dependenc e). We employ the QCE(0) model to illuminate the interplay between mac roscopic and microscopic properties (dependence on particular cluster species, cooperative binding energetics, vibrational entropy factors, etc.) in a purely ab initio framework. We then introduce two simple co rrections for the neglected cluster-cluster interactions: (i) a mean-f ield correction for residual inter-cluster attractions and (ii) a van der Waals-like correction for excluded volume effects. The numerical a ccuracy of the resulting QCE model is demonstrated for a wide range of thermodynamic properties of water, complementing recent comparisons w ith experimental quadrupole coupling data. Some shortcomings of the cu rrent implementation are noted, and prospects for future QCE applicati ons to more complex liquids and liquid mixtures are discussed. (C) 199 8 American Institute of Physics.