Quadratic Potential Function for Ammonia

Abstract

It is found that the Urey‐Bradley force field does not adequately account for the harmonic frequencies or the quadratic potential function of ammonia. The reason for this is that the Urey‐Bradley force field does not account properly for the interactions between the valence‐bond stretching and angle bending displacements in the potential function. A simple model containing four adjustable parameters is proposed for the potential function of ammonia that reproduces both the harmonic frequencies and the quadratic potential function quite accurately.