Beyond the potential energy surface: Ab initio corrections to the Born-Oppenheimer approximation for H2O

It is customary when computing re-vibrational transitions in molecules to i nvoke the Born-Oppenheimer separation between nuclear and electronic motion . However, it is known from accurate calculations on H-2(+) and H-2 that th e first-order (diagonal adiabatic) and second-order (nonadiabatic) correcti ons are not negligible and are both important. In the present work, we have made an ab initio implementation of the Bunker and Moss formalism for the nonadiabatic correction and applied it to H-2 and H2O. From comparison to a ccurate calculations for H-2, we find that we can obtain good results for t he nonadiabatic correction using Cl singles to treat the electronically exc ited states if we scale the results, but we must go beyond the SCF approxim ation to obtain an accurate diagonal adiabatic correction. For H2O, we find that the first-order correction is more important than the second-order co rrection for bending energy levels, but the second-order correction is more important than the first-order correction for stretching energy levels. Th e correction to rotational levels is also significant. Thus, first- and sec ond-order corrections are vital for accurate ab initio predictions of trans ition frequencies.