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

Molecular mechanical force fields are widely used in molecular simulation s tudies 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 th ese, 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 t o changes in its environment. There are several ways of including polarizat ion effects in force fields but, in this paper, we investigate a relatively new approach, called the chemical potential equalization model, which is b ased upon the principle of electronegativity equalization and which is a ge neralization of the class of fluctuating charge models. We detail the princ iples behind the model and of our implementation of it and then present par ametrizations of the model for the molecules, methane and water, and the ch loride anion. Gas- and solution-phase calculations to test the parametrizat ions indicate that the model gives results that are in good agreement with experiment and that compare well to those obtained with non-polarizable for ce fields and other polarizable models.