(139c) Molecular Simulation Of Non-Additive Influences On The Properties Of Water

The key role played by water in many important biological, chemical and physical processes[1-3] has motivated many theoretical and modeling studies. The traditional approach of developing equations of state[4] has been of limited success for water. Accurate reference equations[5] for pure water have been developed, which cannot be easily extended to mixtures. In contrast, theoretical equations of state can often qualitatively predict[6,7] the properties of aqueous mixtures up to very high pressures but they are not reliable for accurate predictions. Conclusions reached from equation of state calculations are generally of limited value because of uncertainties in the theoretical representation of the underlying model and the need to fit equation of state parameters to experimental data.

When used properly, molecular simulation[8] is a useful alternative to the conventional theoretical approach, because it provides unambiguous information regarding the merit of the underlying intermolecular potential used to describe molecular interactions. There are many alternative intermolecular potentials[9] for water, which reflects the difficulty[10] of accurately predicting all the diverse properties of water. Currently, fully ab initio models do not generally provide accurate predictions and the most widely used9 models are variants of either the four-site (TIP4P) or the three-site simple point charge (SPC, SPC/E) models. Extensive investigations of these models indicate that, although they are reasonably accurate at ambient conditions, they show systematic deviations from experiment with increasing temperature.

In this work, the role of non-additive interactions on the structure, dielectric properties and phase behavior of water is investigated at different temperatures using molecular simulation. A new intermolecular potential is developed which contains an ab initio description of two-body additive interactions plus non-additive contributions from both three-body dispersion interactions and polarization. Polarization is the main non-additive influence, resulting in improved agreement with experiment for the radial distribution function, dielectric constant and dipole moment. A comparison is also made with other widely used intermolecular potentials. The new potential provides a superior prediction of the dielectric constant and dipole moment. Significantly, the new potential also predicts the relative contribution of hydrogen bonding better than existing models. Very good agreement with experiment is obtained for the vapor-liquid phase envelope, demonstrating the important influence of non-additive interactions.

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