ABSTRACT
P. R. Bunker, J. W. C. Jensen, and coworkers have developed a variational method to treat the Renner-Teller effect. The Renner-Teller effect has been approached by ab initio methods. Ab initio potential energy surfaces are employed in their calculations, sometimes adjusted to fit experimental data, since at that time ab initio calculations of potential energy surfaces could not be performed to the required accuracy. An extension, named stretch-bender approach, has been developed to include the symmetric and antisymmetric stretch vibrations. To derive the stretch-bender Hamiltonian, the cartesian coordinates of the three atoms are expressed in terms of the coordinates of a reference configuration and displacements. The reference configuration is chosen, such as to follow the minimum path on the potential surface as the molecule bends. Pioneering work was initiated by Carter and Handy in the 1980s. They developed a variational Renner-Teller code that uses the exact kinetic energy operator in valence coordinates.
