Reaction path power series analysis of NH₃ inversion

A new tool for reaction-path analysis of chemical dynamics is introduced, namely, series expansion of reaction-path potentials. This analysis involves the expansion of vibrational frequencies in powers of a reaction coordinate measuring progress along a suitably defined reaction path. The method is applied to generalized-normal-mode vibrational frequencies along two choices of reaction path for the inversion of ammonia. The leading nonzero terms in the power series are calculated by Møller-Plesset second-order perturbation theory and are fully converged with respect to basis set and the step size used for calculating the minimum energy path and the Hessian.