AMBER-ii: New Combining Rules and Force Field for Perfluoroalkanes.

A molecular mechanics force field of the AMBER/OPLS family for perfluoroalkanes, noble gases, and their mixtures with alkanes has been proposed. We had to abandon the traditional Lorentz-Berthelot combining rules for the Lennard-Jones potential to be able to uniformly describe these substance classes and their mixtures. Instead, the Waldman-Hagler rules developed for noble gases were used for all of these elements except hydrogen. Hydrogen is considered to be a particular substance to which the usual Lorentz-Berthelot rules are applied. The proposed rules have little effect on the organic chemistry of H, C, N, and O elements but make it compliant with the chemistry of heavy elements. Because of assigning a relatively high partial charge of -0.37e to fluorine atoms, the new force field reproduces the mutual insolubility of higher liquid alkanes and perfluoroalkanes.