Polarizable empirical force field for aromatic compounds based on the classical drude oscillator.

The polarizable empirical CHARMM force field based on the classical Drude oscillator has been extended to the aromatic compounds benzene and toluene. Parameters were optimized for benzene and then transferred directly to toluene, with parameters for the methyl moiety of toluene taken from the previously published work on the alkanes. Optimization of all parameters was performed against an extensive set of quantum mechanical and experimental data. Ab initio data was used for determination of the electrostatic parameters, for the vibrational analysis, and in the optimization of the relative magnitudes of the Lennard-Jones parameters. The absolute values of the Lennard-Jones parameters were determined by comparing computed and experimental heats of vaporization, molecular volumes, free energies of hydration, and dielectric constants. The newly developed parameter set was extensively tested against additional experimental data such as diffusion constants, heat capacities at constant pressure, and isothermal compressibilities including data as a function of temperature. Moreover, the structures of liquid benzene, liquid toluene, and solutions of each in water were studied. In the case of benzene, the computed and experimental total distribution function were compared, with the developed model shown to be in excellent agreement with experiment.