The nuclear degrees of freedom R = (r, θ, φ), are the bond-length-alternation stretching (r ), the torsional deformation around the double reactive bond C=C (θ) and the hydrogenout-of-plane wagging (φ). These modes drive the ultrafast isomerization of PSB3 after photo-excitation to the first singlet excited state (S 1 ) . The expression for the diabatic potentials and couplings can be found in Appendix A 2.