Title screen

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 Pade-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 χ = 1.8 for 422 levels up to J, Ka ≤ 20 and χ = 2.1 for all available levels up to J ≤ 35. New ground state data reported by Toth (J. Opt. Soc.B8,2236-2255 (1991)) are fitted up to J, Ka ≤ 10 with RMS = 4 × 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