CASSCF/CASPT2 calculation of the low-lying electronic states of the CH3Se neutral radical and its cation.

Electronic states of the CH(3)Se and its cation CH(3)Se(+) have been studied using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with the ANO-RCC(TZP) basis set. To investigate the Jahn-Teller effect on the CH(3)Se radical, C(s) symmetry was used for CH(3)Se in calculations. The results show that the Jahn-Teller effect is very small (69 cm(-1)) and the 1(2)A' state is slightly more stable than the 1(2)A'' state (8 cm(-1)). The CH(3)Se has been found to have a 1(2)A' ground state with a C-Se bond distance of 1.975 A. The computed C-Se stretching nu(6)(a') frequency is 554.1 cm(-1), which is in good agreement with the experimental values of 600 +/- 60 cm(-1). The calculations for CH(3)Se at 3.621 and 5.307 eV are attributed to 1(2)A' --> 2(2)A'(1(2)A(1)) and 1(2)A' --> 2(2)A'', respectively. The vertical and adiabatic ionization energies were obtained to compare with the PES data.