The calculation of spectroscopic Jahn-Teller parameters by ab initio methods

We report a general method for the calculation of Jahn-Teller coupling constants by ab initio methods widely available today in standard packages. The vibrational frequencies corresponding to those obtained experimentally are calculated at the symmetric position using a generalized restricted Hartree-Fock (GRHF) wavefunction. The energy of the symmetric configuration is calculated as a conical intersection using a complete active space self-consistent field (CASSCF) wavefunction. The energy of the distorted configuration is calculated using the same CASSCF active space and occupations. The difference in energy of these two CASSCF calculations is the Jahn-Teller stabilization energy. In addition to the total energy of the state at the cusp, the conical intersection calculation determines the vector along which the molecule will distort. This vector is projected onto the normal modes of the molecule, obtained via the GRHF calculation, so that estimates of the experimentally observable linear Jahn-Teller coupling constants can be obtained. We also present a method for the calculation of the quadratic Jahn-Teller coupling constants. This approach has been applied to and evaluated for the methoxy family of radicals (CH3O, CF3O, CH3S, and CF3S).