Given an an appropriate simulation algorithm that ensures the adequate sampling of phase space, the ultimate accuracy of molecular simulation is limited primarily by the molecular model, or "force field" used to describe interactions between atoms in the molecules of interest. In this poster, new methods of developing force field parameters, especially partial charge distributions, are described. New force fields are presented for polar molecules such as acetone, chloroform, hydrogen sulfide, sulfur dioxide, dimethyl ether and methanol. Vapor-liquid coexistence curves are determined for the above molecules with histogram-reweighting Monte Carlo simulations in the grand canonical ensemble. A similar methodology is used to determine the pressure-composition behavior for various binary mixtures. Overall, the present force fields show an improvement over existing models with respect to the prediction of binary vapor-liquid equilibria, with no loss in accuracy in the prediction of pure component properties.