An ab initio investigation of the potential function and rotation–vibration energies of NH3

Fifty-four points on the potential energy surface of the ground electronic state of the ammonia molecule, with energies up to 9700 cm–1 above equilibrium, have been calculated ab initio using the configuration interaction technique. An analytic potential function has been fitted through these points and the rotation–vibration energy levels have been calculated using the nonrigid invertor Hamiltonian. The agreement with experiment is satisfactory enough that the anharmonic force constants obtained can be realistically used to assist in the interpretation of anharmonicity effects in the spectrum. We calculate the equilibrium bond length as 1.016 Å, the equilibrium HNH angle as 106.2°, and the invertion barrier as 1995 cm−1.