Vibration-rotation levels of water beyond the Born-Oppenheimer approximation

The value of the adiabatic correction to the Born-Oppenheimer electronic energy is calculated as a function of geometry for water using SCF wavefunctions. A mass-dependent adiabatic function is combined with high-accuracy ab initio electronic structure calculations due to Partridge and Schwenke. Vibrational band origins for H2O, D2O, T2O, HDO, HTO and DTO are analysed. Unlike previous calculations on the H-3(+) system, it is suggested that non-adiabatic effects are more important than adiabatic ones in determining accurate isotope dependence of the vibrational band origins of water. Use of the adiabatic surface and effective masses of the heavy particles intermediate between the nuclear and atomic masses is found to significantly improve predictions of rotational term values. The adiabatic correction is found to be of particular importance for rotational levels with high K-a.