Quadratic Potential Function for Ammonia

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.