First principles prediction of isotopic shifts in H2O

We compute isotope-independent first- and second-order corrections to the Born-Oppenheimer approximation for water and use them to predict isotopic shifts. For the diagonal correction, we use internally contracted multireference configuration interaction wave functions and derivatives with respect to mass-dependent internal coordinates to generate the mass-independent correction functions. For the nonadiabatic correction, we use a self-consistent field wave function for the ground electronic state and single excitation configuration interaction wave functions for the excited states and a generalization of the Handy, Yamaguchi, and Schaefer method to obtain mass-independent correction functions. We find that including the nonadiabatic correction gives significantly improved results compared to just including the diagonal correction when the Born-Oppenheimer potential energy surface is optimized for (H2O)-O-16. The agreement with experimental results for deuterium- and tritium-containing isotopes is nearly as good as our best empirical correction, however, the present correction is expected to be more reliable for higher, uncharacterized, levels. (C) 2003 American Institute of Physics.