Sampling averages from the (each) 5000 pathways collected for the S0-Z and S0-E starting points, respectively, we produced time-dependent profiles of the -C=C-torsion angle (fig. , see also supplementary information S3.1). The different relaxation dynamics are best seen from contrasting S0-Z/E → S1 in vapor to HTI photoswitching in solution, respectively. In the former, 0/180° → 90° deformation of the -C=C-torsion angle is observed on the fs scale, however leading to strong vibrations of the entire HTI molecule -and thus re-visiting the Z/E type configurations. In turn, after energy dissipation from the torsion degree of freedom to molecular vibration, we find convergence to the S1 state within about 10 ps. On the other hand, in solution the torsional motion is slowed by nearby solvent molecules. This damps HTI vibrations significantly and convergence to the equilibrium S1 state is found at the 5 and 15 ps scale in hexane and DMSO, respectively. Statistics of the vertical energy difference between the S0-and the S1-state are reported in the supplementary information S3, whereas the electronic decay from S1-states is discussed in the following. From comparing the evolution of photoswitching pathways starting from S0-Z and S0-E, we argue that upon 20 ps propagation in the S1 state, our MD simulations converged to S1 type configurations. Using the endpoints of these 2×5000 trajectories, we switched our MM model back to the S0 state to mimic the electronic decay to the ground state. In all solvent scenarios, we find the HTI molecules to lock into either S0-Z or S0-E configurations within only a few picoseconds of NVE simulation runs (fig. ). While evolution to the S0-Z state is preferred over all solvent scenarios, the observed Z/E ratio after completing the photoswitching cycle differs significantly. Indeed, the probability of finding S0-Z conformers (which was found as 55. collected for the S1→S0 transition runs. To this end, the probability of HTI relaxation to the E-isomer p(S1→S0-E) is assumed as the reaction coordinate. While mathematically exact, it however provides little help for our actual understanding of the underlying mechanisms. This motivates the formulation of structural descriptors of which we created a small series of candidates from intuition.