Information about the model used in our simulations is given in Appendix A. The three electronic states, the ground and the two excited singlet states will be indicated as S 0 , S 1 , and S 2 respectively. A ball-and-stick representation of phenol is shown in Fig. potential energy surfaces (PESs) are represented in Fig. as functions of the OH bond length r and of the angle θ formed by the OH bond with the plane of the cycle. The figure shows that S 0 and S 1 have a dissociative character for large values of r , thus hydrogen dissociation takes place either in the ground state or in the first excited state. The photochemistry of phenol is activated by exciting the molecule from the ground state S 0 (blue surface) to the bright state S 1 (orange surface), since in the Franck-Condon region, i.e., around the minimum of S 0 where the photo-excitation process starts, the S 1 state has a ππ * character. The inset in Fig. shows, using a cut of the PESs at θ = 0, the diabatic potential curves (PECs) with the states labelled as in Appendix A. The diabatic PECs clearly show that the minimum of the bright ππ * state (green curve) almost coincides with the minimum of the ground state (red-violet curve) and that the closest degeneracy point (conical intersection between S 1 and S 2 ) between the ππ * state and the πσ * state (dark state, turquoise curve) is higher in energy than the value of the ππ * energy at the Franck-Condon point, i.e., the minimum of the ground state. This means that upon photo-excitation of the vibrational ground state from S 0 to S 1 , the hydrogen dissociation cannot be activated (not even considering the zero point energy): if the dark state πσ * is not populated, then the system cannot reach the dissociative regions of the potentials and remains trapped in the minimum of the ππ * state. In Ref. [62], in fact, an excited vibrational state of S 0 was promoted to S 1 in order to activate the photochemical reaction. Due to the difficulties in creating a vibrational excited state using a classical distribution of trajectories, we opted in our simulations to provide the necessary energy to reach the conical intersection by adding an additional momentum p 0 in the r direction. Indeed, the quantum dynamics results used as benchmark here are obtained in the same conditions. Computational details are provided in Appendix B.