At later times, t = 50 fs in Fig. (central panel), the nuclear density starts spreading symmetrically towards the CIs as consequence of the appearance of a saddle shape along ϕ with maximum in ϕ = 0 in the region q ∈ [-3, 0]. In the regions q < -3 and q > 0, the increase of the TDPES localizes the nuclear density along the q direction. In particular, the shape of the TDPES around ϕ = 0, q = 2 indicates the presence of a small portion of the nuclear density in the ground state. We recall that quantum dynamics is performed in the diabatic basis and, thus, the nuclear state is initialized with full population in the diabatic state k = 1. This means that a very small amount of density is found in the (adiabatic) ground state even at time t = 0, whose evolution is also dictated by the TDPES. In the portion of configuration space q > 0 at t = 50 fs the (adiabatic) excited-state density is much smaller than the ground-state density, thus the TDPES follows the shape of the S 0 PES.