An accurate ab initio quartic force field for ammonia

Abstract

The quartic force field of ammonia is computed using basis sets of spdf/spd and spdfg/spdf quality and an augmented coupled cluster method. After correcting for Fermi resonance, the computed fundamentals and ν4 overtones agree on average to better than 3 cm−1 with the experimental ones except for ν2. The discrepancy for ν2 is principally due to higher-order anharmonicity effects. The computed ω1, ω3, and ω4 confirm the recent experimental determination by Lehmann and Coy but are associated with smaller error bars. The discrepancy between the computed and experimental ω2 is far outside the expected error range, which is also attributed to higher-order anharmonicity effects not accounted for in the experimental determination. Spectroscopic constants are predicted for a number of symmetric and asymmetric top isotopomers of NH3.