x 1 = 0.5, x 8 = 1.0, x 23 = 1.5. The figure highlights with the black and gray dashed lines the displacement of the position of the CI along the ϕ coordinate. The evolution of the Gaussian nuclear wavepacket -prepared in S 0 before photo-excitation -is initiated by an instantaneous S 0 -to-S 1 transition. Its expression is