THE ADIABATIC ROTATION APPROXIMATION FOR ROVIBRATIONAL ENERGIES OF MANY-MODE SYSTEMS - DESCRIPTION AND TESTS OF THE METHOD

We extend the vibrational self-consistent field method (VSCF), and two types of state mixing [denoted VSCF-CI and V-CI (configuration intera ction)]: to include an approximate, adiabatic treatment of overall rot ation. In this approach, the asymmetric-top rotational Hamiltonian is diagonalized in an ''instantaneous'' principal axis system, and the re sulting coordinate-dependent rotational energy is added to the exact H amiltonian of the nonrotating system to form an effective Hamiltonian for the rotation/vibrational energies. The energy eigenvalues of that Hamiltonian are then obtained by the VSCF approach and/or variational, stare-mixing methods. In this present formulation for many-mode syste ms, we use the general Watson Hamiltonian, and also a hierarchical rep resentation of the many-mode potential described previously [S. Carter , S. Culik, and J. M. Bowman, J. Chem. Phys. 107, 10458 (1997)]. This approach, at the VSCF, and VSCF-CI and V-CI levels is tested against r ecent exact calculations of vibrational/rotation energies of HO2 and H 2O. HO2 is an approximate prolate symmetric top, which is a favorable case for the approximate treatment of rotation, whereas H2O is a highl y asymmetric top with large rotation constants, and represents an unfa vorable case for the method. (C) 1998 American Institute of Physics.