CALCULATION OF HIGH ROTATION ENERGIES OF THE WATER MOLECULE USING THE GENERATING FUNCTION MODEL

The previously developed generating function model is applied in this paper to fitting high rotational levels of the water molecule in the domain of rotational quantum numbers where the standard power series expansion of the rotational Hamiltonian has a slow rate of convergence or even diverges. This model typically provides a considerable improvement in the standard deviation of fit with respect both to the conventional polynomial Hamiltonian and to the Fade-type Hamiltonian (with the same or fewer number of adjustable parameters). Ground state rotational levels recovered by Flaud et al. (Mol. Phys. 32, 499-521 (1976)) from flame spectra are fitted with accuracy near that of the experimental values: weighted standard deviation chi = 1.8 for 422 levels up to J, K-a less than or equal to 20 and chi = 2.1 for all available levels up to J less than or equal to 35. New ground state data reported by Toth (J. Opt. Sec. B 8, 2236-2255 (1991)) are fitted up to J, K-a less than or equal to 10 with RMS = 4 X 10(-5) cm(-1). Tests of extrapolation properties of the generating function model are discussed. In certain cases the accuracy of extrapolation was better than the accuracy of fitting with the standard rotational Hamiltonian. (C) 1995 Academic Press, Inc.