Renner–Teller effect in six-atomic molecules: Ab initio investigation of the vibronic spectrum of C6-

In this study we introduce a model for handling the Renner–Teller effect in Π electronic states of six-atomic molecules with linear equilibrium geometry. The applied model Hamiltonian excludes the stretching vibrations and end-over-end rotations. On the other hand, it considers the interplay between the vibronic and spin–orbit couplings. We derive perturbative formulae for important coupling cases and apply them to compute the structure of the vibronic spectrum of the X2Πu state of the C6- ion. The corresponding potential energy surfaces are computed by means of a DFT approach, and the spin–orbit coupling constant by the SA-CASSCF+MRCI method.