Use of second-order perturbation theory for the vibrational energy levels and partition functions at a saddle point

Recent ab initio information of Kraka and Dunning on the saddle point region for the reaction OH+H2→H2O+H is used to construct an anharmonic potential energy surface valid near the saddle point. Anharmonic vibrational energy levels involving the bound degrees of freedom orthogonal to the reaction coordinate at the saddle point are obtained using perturbation theory through second order for cubic terms and first order for quartic terms, with resonance effects removed. These energy levels are compared to those obtained from an accurate self-consistent field configuration-interaction method, and are used to calculate thermal vibrational partition functions over the temperature range from 200 to 2400 K.