Central-force model for liquid water

Abstract

We propose a new class of Hamiltonian models of liquid water based on resolution of the monomeric unit into three effective point charges. Interacting through central forces only, the three charges automatically assume the molecular structure. Two important effects are built into this model which have been neglected in similar attempts: intramolecular modes of vibration and the capacity for self−dissociation in the liquid phase. In addition, a large number of microsopic properties of water can be expressed in very simple terms for this representation: The pressure and internal energy, second virial coefficient, high−frequency elastic moduli, and dielectric function are discussed in explicit terms. A convenient algorithm for computing low−order quantum corrections (proportional to h/2) to thermodynamic properties is given as well. To illustrate the general class of central−force models, we provide a concrete realization which has been determined by fitting phenomenological potentials to a nearly linear hydrogen bond of proper energy and dimer configuration. In order to elucidate the microscopic consequences of assuming central−force interactions in water, we have investigated the energy variation of small polymers (dimers and trimers) and the solvated proton near their minimum energy configurations. On a qualitative level, the results of these initial computations provide considerable encouragement for the view that water molecule interactions can be realistically approximated by linear combinations of central forces.