Evaluating free energy, enthalpy, and entropy of protonated water clustersby a grand canonical Monte Carlo simulation

We report the results of a Monte Carlo simulation of ion clusters using the polarizable model potentials for hydronium ion and a water molecule propos ed by Kozack and Jordan [J. Chem. Phys. 96, 3120 (1992); 96, 3131 (1992)]. The grand canonical Monte Carlo simulation aided by the umbrella sampling t echnique allows us to evaluate very efficiently the equilibrium distributio n of ion clusters of various sizes. Thermochemical data of interest, such a s the standard free energy, enthalpy, and entropy of protonated water clust ers follow immediately from this distribution. (C) 2000 American Institute of Physics. [S0021- 9606(00)51541-4].