A new electrostatic model, designed for use in molecular mechanics force fi
elds, is presented. In addition to atomic charges the model consists of ato
mic dipoles and is further enhanced by the possibility of explicitly accoun
ting for polarizability in the form of induced charges and anisotropically
induced atomic dipoles. The parameters of the model are determined in a con
sistent way from isolated molecules by fitting to ab initio electric potent
ials. The polarizability in different directions is probed by subjecting th
e molecules to various external electric fields. As an illustration of the
model. parameters have been obtained for isolated water and formaldehyde mo
lecules, and applied to a few hydrogen-bonded water dimers and water-formal
dehyde complexes. The parameters transferred from the isolated molecules su
ccessfully reproduce the ab initio (MP2/6-31 ++G(d,p)) electric potentials,
molecular dipole moments. and molecular polarizabilities of all the studie
d 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.