Self-Consistent Field (SCF) and Configuration Interaction (CI) studies are performed on the bending mode of the water molecule using a double zeta plus polarization basis set. The ab initio points are fitted to a three-parameter double minimum potential consisting of a quadratic plus Lorentzian terms. The vibration-rotation energies are then evaluated using the large amplitude Hamiltonian developed by P. R. Bunker and co-workers at various levels of approximations. It is found that the calculated frequencies improve significantly as one proceeds from approximate H b 00( ρ) to rigid bender H b 0( ρ) [P. R. Bunker and J. M. R. Stone, J. Mol. Spectrosc. 41, 310–332 (1972)] to semirigid bender H b 0( r, ρ) [P. R. Bunker and P. M. Landsberg, J. Mol. Spectrosc. 67, 374–385 (1977)] Hamiltonian. With H b 0( r, ρ), the ab initio calculated bending frequency ν 2 differs from the observed value (1595 cm −1) by 30 cm −1 and the barrier height is 12 229 cm −1. It is also shown that ν 2 and its first four overtones are better calculated by 45–98 cm −1 when the ab initio potential is used directly instead of the three-parameter analytic potential fitted to ab initio data. Finally, rotation bending energy levels are calculated for v 2 ≤ 3 and J ≤ 10 on the basis of a nonrigid bender Hamiltonian of A. R. Hoy and P. R. Bunker [ J. Mol. Spectrosc. 74, 1–8 (1979)], using the ab initio quadratic force field of P. Hennig, W. P. Kraemer, G. H. F. Diercksen, and G. Strey, [ Theor. Chim. Acta 47, 233–248 (1978)]. These results show that the accuracy of calculated force constants and frequencies is critically dependent not only on the size of the basis set but also on the number and spacing of the ab initio points used to derive the force field.
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