Vibrational energy levels of water

Several quartic force fields and a full sextic anharmonic force field for H 2 O have been determined from high-quality ab initio calculations, the highest at the aug-cc-pVQZ CCSD(T) level of theory. These force fields have been used to determine vibrational excited state band origins up to 15000 cm −1 above the zero-point level, using both a perturbation-resonance approach and a variational approach. An optimised quartic force field has been obtained by least squares refinement of our best ab initio results to fit the observed overtone levels of 5 symmetrically substituted isotopomers of water (H 2 16 O, H 2 17 O, H 2 18 O, D 2 O, and T 2 O) with an rms error of less than 10 cm −1 , using the perturbation-resonance model for the vibrational calculation. Predicate least squares refinement was used to provide a loose constraint of the refined force field to the ab initio results. The results obtained prove the viability of the perturbation-resonance model for use in larger molecular systems and also highlight some of its weaknesses.