Intermultiplet interactions in normal and local mode molecules in the algebraic force-field expansion approach

The algebraic force-field expansion recently proposed [T. Sako, K. Yamanouc hi, and F. Iachello, Chem. Phys. Lett. 299, 35 (1999)] is applied to fit th e experimental vibrational term values of H2O and SO2 in the electronic gro und (X) over tilde (1)A(1) state. The comparison of results of least-square s fits by the algebraic force-field expansion with those by the conventiona l force-field expansion shows that the convergence of the algebraic model i s much faster than that of the conventional model and this rapid convergenc e becomes more significant when the Hamiltonian is expressed in local coord inates rather than in normal coordinates. It is also demonstrated that coor dinate-space vibrational wave functions can be constructed directly through the experimental-level energy fit by the algebraic Hamiltonian expansion. From the nodal patterns of the vibrational wave functions constructed with the optimized Hamiltonian parameters of SO2, the bifurcation of the wave fu nctions characteristic of the local-mode doublet states are identified in a vibrationally highly excited energy region. It is shown that the local-mod e structure of the normal-mode limit molecule SO2 has the same origin as th at for the local-mode limit molecule H2O. (C) 2000 American Institute of Ph ysics. [S0021-9606(00)01141-7].