The force fields most widely used today approximate the electrostatic interactions of a molecular system in terms of Coulombic interactions among fixed, atom-centered, partial charges. The partial charges of a given molecule are typically chosen to replicate the electrostatic potential (ESP) generated by the molecule in vacuum, as computed by reference quantum mechanical (QM) methods. Such so-called fixed charge electrostatic models are computationally efficient and have been remarkably useful. However, their accuracy is limited by their neglect of the field-dependent electronic polarization, i.e., of changes in the molecule's electron distribution in space in response to changes in an electric field.