Product Basis Set Variational Calculations of Triatomic Rovibrational Energy Levels and Wavefunctions

Abstract

If both the potential energy surface of a molecule and trial functions for a variational calculation are expanded in products of internal coordinate functions, the Hamiltonian matrix may be speedily built because all required integrals are one-dimensional. New methods, especially for calculating potential energy matrix elements, have dramatically decreased the time required for the matrix build, and have also improved the accuracy of the matrix elements.

Variational calculations have been performed to the Murrell, Carter, and Halonen (J. Mol. Spec. 93, 307 (1982)) potential energy surface for HCN, a molecule with a strong bend-stretch coupling and an HCN/HNC isomerization. States without high bend excitation or localized to the HCN side of the potential energy surface converged well. Highly excited HNC states were more difficult to converge. One difficulty in our calculations was that the fit had several holes (large negative regions) outside the range of fitting, causing spurious energy levels. Several methods for dealing with these holes during wavefunction calculations are described.