The potential energy surface for the electronic ground state of the water molecule determined from experimental data using a variational approach

Abstract

In the present work, the MORBID (Morse oscillator-rigid bender internal dynamics) computer program [P. Jensen, J. Mol. Spectrosc. 128, 478–501 (1988)] has been used to optimize the parameters in an analytical expansion of the potential energy function for the electronic ground state of the water molecule. The input data for the fitting consisted of 550 experimental energy separations involving rotation-vibration energy levels with J ≤ 2 in 103 vibrational states [with energies up to 19 000 cm −1 above the (0, 0, 0) state] belonging to the six isotopic molecules H 2 16O, D 2 16O, HD 16O, H 2 17O, and T 2 16O. In the fitting 19 parameters were varied, and the standard deviation was 0.63 cm −1. In contrast to conventional methods for calculating potential energy derivatives (force constants) from experimental data [A. R. Hoy et al., Mol. Phys. 24, 1265–1290 (1972)] the MORBID approach does not make use of perturbation theory. The potential energy surface obtained is compared with the results of conventional force field calculations [I. M. Mills, in “Specialist Periodical Reports, Theoretical Chemistry” (R. N. Dixon, Ed.) Vol. 1, The Chemical Society, London, 1974] and recent variational calculations by S. Carter and N. C. Handy [ J. Chem. Phys. 87, 4294–4301 (1987)] and by L. Halonen and T. Carrington, Jr. [ J. Chem. Phys. 88, 4171–4185 (1988)].