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Explicit polarization theory

Molecular mechanical force fields have been successfully used to model condensed-phase and biomolecular systems for a half century. Molecular mechanical force fields are analytic potential energy functions based on classical mechanical force constants, van der Waals potentials, electrostatics, and torsional potentials, with parameters fit to experiment, to quantum mechanical calculations, or to both. Accurate results can be obtained from simulations employing molecular mechanics for processes not involving bond breaking or bond forming. In this article, we describe a new approach to developing force fields; this approach involves the direct use of quantum mechanical calculations rather than using them as a training set for classical mechanical force fields. Computational efficiency is achieved by partitioning of the entire system into molecular fragments. Since the mutual electronic polarization is explicitly treated by electronic structural theory, we call this approach the explicit polarization (X-Pol) method. Strategies and examples are presented to illustrate the application of X-Pol to describe intermolecular interactions as a quantum chemical model and as a force field to carry out statistical mechanical Monte Carlo and molecular dynamics simulations.