Ab Initio MBPT(4) Calculations of the Inversion Potential Function of NH3

The inversion potential function and the dominant (in-plane) and inversion-(in-plane) force constants are calculated by means of the fourth-order many-body perturbation theory in the extended double-zeta-plus polarization basis set. Using the ab initio potential surfaces and the non-rigid invertor Hamiltonian the inversion energy levels of 14NH3 are calculated. Despite the significant residual error in the perturbation expansion, the results indicate that the full fourth order procedure may be considered as a reliable tool for studying vibrational anharmonicity in other non-rigid molecules