Development of the trappe force field for ammonia

The transferable potentials for phase equilibria (TraPPE) force field is extended through the development of a non-polarizable five-site ammonia model. In this model, the electrostatic interactions are represented by three positive partial charges placed at the hydrogen position and a compensating partial charge placed on an M site that is located on the C₃ molecular axis and displaced from the nitrogen atom toward the hydrogen atoms. The repulsive and dispersive interactions are represented by placing a single Lennard-Jones site at the position of the nitrogen atom. Starting from the five-site model by Impey and Klein (Chem. Phys. Lett. 1984, 104, 579), this work optimizes the Lennard-Jones parameters and the magnitude of the partial charges for three values of the M site displacement. This parameterization is done by fitting to the vapor-liquid coexistence curve of neat ammonia. The accuracy of the three resulting models (differing in the displacement of the M site) is assessed through computation of the binary vapor-liquid equilibria with methane, the structure and the dielectric constant of liquid ammonia. The five-site model with an intermediate displacement of 0.08 Å for the M site yields a much better value for the dielectric constant, whereas differences in the other properties are quite small.