Supplementary Material

Pseudo-Jahn-Teller effect; derivation of vibronic coupling density; benchmarking of functionals for calculation of absorption spectrum; optimized structure of C6-; normal vibrational modes of C6-; Franck-Condon states of C6-; Franck-Condon transitions associated with non-totally symmetric modes; the PJT effect in potential softening in the C2Πg state along the ν9(πu) mode; frequency change by QVCC difference; electronic configurations of C6-; QVCD analysis for the C2Πg state and the π modes of C6-; adiabatic excitation energy of the D2Πg and E2Πg states.