A new electrostatic model for molecular mechanics force fields

A new electrostatic model, designed for use in molecular mechanics force fields, is presented. In addition to atomic charges the model consists of atomic dipoles and is further enhanced by the possibility of explicitly accounting for polarizability in the form of induced charges and anisotropically induced atomic dipoles. The parameters of the model are determined in a consistent way from isolated molecules by fitting to ab initio electric potentials. The polarizability in different directions is probed by subjecting the molecules to various external electric fields. As an illustration of the model. parameters have been obtained for isolated water and formaldehyde molecules, and applied to a few hydrogen-bonded water dimers and water-formaldehyde complexes. The parameters transferred from the isolated molecules successfully reproduce the ab initio (MP2/6-31 ++G(d,p)) electric potentials, molecular dipole moments. and molecular polarizabilities of all the studied dimers and complexes. The performance of nonpolarizable models for these systems is also evaluated, and, for tired hydrogen-bond distances, is found to be acceptable in most cases. (C) 2000 Elsevier Science B.V. All rights reserved.