Accurate ab initio quartic force field and vibrational frequencies of the NH4+ ion and its deuterated forms

The quartic force field of NH4+ has been computed using advanced coupled cluster methods and a basis set of spdf quality. Effects of further basis set extension and of the inclusion of core correlation on the geometry and harmonic frequencies have been investigated. Our best computed values are: re = 1.02029 Å, ν1 = 3236.6, ν2 = 1690.1, ν3 = 3345.1, and ν4 = 1447.2 cm−1, in excellent agreement with the experimental data. Complete sets of fundamentals and anharmonicity constants are reported for NH3D+, NH2D2+, NHD3+, and ND4+.