The total molecular dipole moment for liquid water

For the water molecule, the dipole is the first nonzero multipole moment; it represents the polarity of the molecule and has been widely used in describing solvation behavior. A rather wide range of theoretically determined values for the total molecular dipole moment of water in condensed phases has been reported in the literature. This paper describes a means by which the average total dipole moment for the water molecule in the liquid state can be linked to experimental refractive index data. Three components comprise the mean-field approach that is employed. A formal framework is developed that relates the temperature dependence of the effective molecular polarizability to the average local electric field experienced by a liquid water molecule over a chosen temperature range. A characterization of the distributions of local fields and field gradients is also necessary, and this has been determined from the computer simulations of liquid water samples at several different temperatures for two standard water potentials. The final component, the electric response properties of the water molecule (including nonlinear contributions up to fourth order), were determined from ab initio calculations for gas- and liquid-phase molecules, and are reported elsewhere [A. V. Gubskaya and P. G. Kusalik, Mol. Phys. 99, 1107 (2001)]. By combining these three components, the temperature dependence of the average local electric field, and consequently the average total dipole moment, are extracted from data for the refractive index of liquid water. An almost 10% variation in the dipole moment with temperature is observed over the range 273 to 373 K. The value obtained for the molecular dipole moment at 300 K, 2.95±0.2 D, is in excellent agreement with a recently reported result extracted from x-ray scattering data, as well as with some recent theoretical predictions.