A significant fraction of the non-pairwise additive part of the interaction energy between water molecules arises from three-body exchange and polarization forces. While most water models incorporate many-body forces in some average manner, a few (called polarizable force fields) describe only the many-body polarization forces but neglect the nonadditive exchange forces. The goal of this work is to understand the possible role of the neglected exchange forces. We also seek to determine the true effect of the many-body polarization forces, as represented by the Drude model, in describing liquid water. Thus, we have taken an accurate ab initio two-body pairwise potential of water that is able to quantitatively reproduce experimental dimer spectra, and coupled it with a Drude model which describes the polarization forces. We show that the use of the Drude model alone omits about 50% of the three-body forces (the part that arises from the exchange interaction) but, despite this omission, the properties of liquid water predicted by the two-body plus polarization model are in reasonable agreement with experimental values in the temperature range from �0-40�C, especially when compared with the results obtained from the use of the pure two-body pairwise potential. Finally, we examine the `tetrahedrality' of the water structure by computing an appropriate order parameter with and without the explicit polarization model.

How well can the properties of liquid water be described without the inclusion of many-body exchange forces?

247th National Meeting (2014)

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