Calculations are presented for vibrational energy levels of nonbending H2O (at correct and linear geometries) and for linear HDO, DTO, C16O2, and C16O18O. Results obtained from accurate numerical studies are compared to those generated by using the self-consistent-field (SCF) approximation in both normal and local coordinates. We find that for all cases except H2O, SCF corrections systematically and considerably improve the zero-order uncoupled normal-mode eigenvalues and render them superior to the local modes even for part of the vibrational overtone region of HDO and DTO. For H2O, the local-mode picture remains better than SCF, due partly to the small size of the Wilson coupling in local modes (or equivalently to the near-degeneracy of the normal modes). These results demonstrate the quality of the SCF as the best independent-mode approximation. Moreover, unlike the cruder decoupled-mode results, the SCF frequencies are in many cases of nearly spectroscopic accuracy.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry