A theoretical study of the Renner–Teller effect in the X̃2Πg state of C3−

Three-dimensional potential energy functions for the A″ and A′ components of the X̃2Πg state of C3− have been generated ab initio using the internally contracted multi-reference configuration interaction method. The spin–orbit constant in this state has been calculated from a complete-active-space-self-consistent-field wavefunction. These data were used to solve the linear/linear Renner–Teller problem variationally for J = 1/2, 3/2 and 5/2. Rovibronic levels for K=0, 1 and 2 were determined for energies up to about 3800 cm−1. The very large splitting between A″ and A′ was found to give rise to a strong mixing for the rovibronic Π and Δ bending states. The results are compared with those obtained experimentally and for similar molecular systems.