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

We report a general method for the calculation of Jahn-Teller coupling cons tants 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 Hartre e-Fock (GRHF) wavefunction. The energy of the symmetric configuration is ca lculated as a conical intersection using a complete active space self-consi stent field (CASSCF) wavefunction. The energy of the distorted configuratio n is calculated using the same CASSCF active space and occupations. The dif ference in energy of these two CASSCF calculations is the Jahn-Teller stabi lization 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 th e molecule, obtained via the GRHF calculation, so that estimates of the exp erimentally observable linear Jahn-Teller coupling constants can be obtaine d. We also present a method for the calculation of the quadratic Jahn-Telle r coupling constants. This approach has been applied to and evaluated for t he methoxy family of radicals (CH3O, CF3O, CH3S, and CF3S).