A chemical potential equalization model for treating polarization in molecular mechanical force fields

Molecular mechanical force fields are widely used in molecular simulation studies due to their simplicity and efficiency. Although accurate enough for many purposes, force fields are approximations and neglect several effects that are important at an atomic level. Probably the most significant of these, that is not taken into account by the majority of force fields, is the ability of the charge distribution of a molecule to polarize in response to changes in its environment. There are several ways of including polarization effects in force fields but, in this paper, we investigate a relatively new approach, called the chemical potential equalization model, which is based upon the principle of electronegativity equalization and which is a generalization of the class of fluctuating charge models. We detail the principles behind the model and of our implementation of it and then present parametrizations of the model for the molecules, methane and water, and the chloride anion. Gas- and solution-phase calculations to test the parametrizations indicate that the model gives results that are in good agreement with experiment and that compare well to those obtained with non-polarizable force fields and other polarizable models.