Fluctuating charge, polarizable dipole, and combined models: Parameterization from ab initio quantum chemistry

We performed studies of fluctuating charge, fluctuating dipole, and combine d models for substituted benzenes and concluded that dipoles are necessary to avoid errors in important cases. Force fields incorporating fluctuating dipoles for alanine, serine, and phenylalanine were developed that accurate ly reproduce both relative conformational energies and cooperative many-bod y energies as given by ab initio quantum chemical calculations. The polariz ation model was fit to quantum chemical calculations of changes in the elec trostatic potential (ESP) induced by applied perturbations. The electrostat ic model was completed by adding fixed charges fit to the zero-field quantu m chemical ESP. All intramolecular and Lennard-Jones parameters, and some t orsional parameters, were taken from the OPLS-AA force field of Jorgensen a nd co-workers. Key torsional parameters were refit to quantum chemical stru ctures and energies.