A SPECTROSCOPICALLY DETERMINED POTENTIAL-ENERGY SURFACE FOR H-3+

A potential energy surface for H+3 is determined by adjusting II parameters from the ab initio surface of Lie and Frye (J. Chem. Phys.96, 6784-6790, 1992) to a data set of 243 H+3 energy levels covering vibrational levels up to 3v2 and rotational levels up to J = 9. The surface is determined by successive refinements using variational calculations to reproduce the experimental data. A method which used cheaper but unconverged calculations for all but the final iteration of the fit is outlined. The resulting potential reproduces the observed data with a standard deviation of 0.053 cm-1. Revised estimates of H+3 band origins up to 5v2 are presented. Predictions for the fundamental frequencies using this potential are good for D+3 but poor for the mixed isotopomers H2D+ and D2H+. It is suggested that these errors, which are about 0.5 cm-1, are due the breakdown of the Born-Oppenheimer approximation. Comparisons are made with known non-Born-Oppenheimer terms for H+2 and H2. © 1994 Academic Press, Inc.