Gas-Phase Molecular Clustering of TIP4P and SPC/E Water Models from Higher-Order Virial Coefficients

Higher-order virial coefficients (up to B6) for TIP4P and SPC/E water models are used to characterize molecular clusters (up to hexamers) formed by water at various gas-phase thermodynamic state points between 298 and 773 K. Comparison of cluster statistics with available molecular simulation data for the same models indicates that the virial approach is effective at characterizing the clustering behavior. Significant deviations from experimentally confirmed ab initio results from the literature at 298 K are ascribed to inadequacies in the TIP4P model and to differences in the treatment of "physical" versus "chemical" association in the two approaches. At two conditions where an analysis could be made, the concentration of clusters that are only physically associated was found to be ~30% of the concentration of those that are chemically associated (hydrogen bonded).