Knowledge Bank

An ab initio potential energy surface for ammonia has been used to calculate vibrational-rotational spectroscopic constants. A complete quartic Taylor series fit in both internal and normal coordinates was derived. Spectroscopic constants were calculated for deuterated isotopomers using normal coordinate force constants, equilibrium geometries, and normal mode eigenvectors within the framework of second-order perturbation theory. Anharmonic spectroscopic constants for ammonia compare favorably with other ab initio calculations. Normal mode force constants are in reasonable agreement with empirically obtained values.