Explicit Polarization Theory

Molecular mechanical force fields have been successfully usedto model condensed-phase and biomolecular systems for a halfcentury. Molecular mechanical force fields are analytic potentialenergy functions based on classical mechanical force constants, vander Waals potentials, electrostatics, and torsional potentials, withparameters fit to experiment, to quantum mechanical calculations,or to both. Accurate results can be obtained from simulationsemploying molecular mechanics for processes not involving bondbreaking or bond forming. In this chapter, we describe a newapproach to developing force fields; this approach involves the directuse of quantum mechanical calculations rather than using them asa training set for classical mechanical force fields. Computationalefficiency is achieved by partitioning of the entire system intomolecular fragments. Since the mutual electronic polarization isexplicitly treated by electronic structural theory, we call thisapproach the explicit polarization (X-Pol) method. Strategies andexamples are presented to illustrate the application of X-Pol todescribe intermolecular interactions as a quantum chemical modeland as a force field to carry out statistical mechanical Monte Carloand molecular dynamics simulations.