The quartic force field of H2O determined by many-body methods. II. Effects of triple excitations

A b initio coupled cluster and many-body perturbation theory methods that include triple excitation effects are applied to the determination of the quartic force field of the water molecule using an extended Slater-type basis set. Predictions of fundamental, overtone, and combination vibrational frequencies, rotational constants, and vibration–rotation coupling constants are reported for H2O and its isotopomers. The best predicted harmonic frequencies for the stretching modes of H2O are accurate to 3 cm−1, while the bending mode has an error of 28 cm−1. The mean absolute error for all frequencies reached by two quanta is 0.6%, while the anharmonic constants xi j have a mean absolute error of less than 3%. The important role of triple excitation effects in the surface determination is discussed, and is compared with the effects of quadruple excitations.