PARAMETERIZING A POLARIZABLE INTERMOLECULAR POTENTIAL FOR WATER

The parameterization of a polarizable intermolecular potential for water is attempted starting from a simple model of three permanent charges, a Lennard-Jones centre on the oxygen, and a single polarizable site on the centre of mass. It is found that the position of the negative charge is a surprisingly critical variable in parameterizing a successful model. The only model that reproduces 1.0 g cm(-3) water properties reasonably well has the negative charge displaced 0.2 Angstrom away from the oxygen along the molecular bisector. This model reproduces the correct energy, pressure, diffusion coefficient, and structure of normal water. At supercritical conditions, however, the model, as with non-polarizable models, does not reproduce these properties satisfactorily; the pressure is too high and the diffusion coefficients are too low. In addition, a comparison with published pair distribution functions shows too much OH structure at the lowest density, whereas at the highest density the opposite is true. There are, however, many new lines of evidence strongly suggesting that the experimental data are affected by some systematic errors, and it is thought that this polarizable model gives a more realistic picture of the structure than non-polarizable models. The model does not employ any short-range damping of the induced dipole. Since short-range damping would tend to decrease the average total dipole moment, its inclusion might improve the diffusional and structural properties at low densities.