In the following, we show that nuclear motion in the lowest MLCT states is non negligible and is in fact the key to the ultrafast 3 MLCT quenching mechanism by 3 MC states. For simplicity, we here analyse the lowest out of the numerous triplet states: Figure shows the largest differences in nuclear gradients at the FC geometry between the lowest four 3 MLCT and three 3 MC states. We here focus on nuclear motion in the MLCT states and its effect on the 3 MLCT-3 MC dynamics, therefore, in Figure , we highlight in red the most significant absolute gradient differences (above 0.02 eV; note that all nuclear gradients are given for unit nuclear displacement expressed in dimensionless normal mode coordinates) for which the 3 MLCT gradient is also significant (at least 0.02 eV). As seen in Figure (red bars), excited-state 3 MLCT nuclear motion will occur along several modes. Importantly, all these modes, with the exception of ν 35 and ν 36 , will reduce the 3 MLCT-3 MC energy gap (∼0.16-0.39 eV for the analysed 4 3 MLCT and 3 MC states, see ESI, Table ), as movement from the FC region towards the 3 MLCT minimum for these modes raises the 3 MC energy (for ν 35 and ν 36 , the opposite effect is observed, i.e., the 3 MLCT-3 MC gap slightly increases upon MLCT geometry relaxation). All 13 iden- tified modes possess dominant ligand ring deformation and/or internal ligand bending character, in accordance with nuclear motion triggered by MLCT excitation (for animation of the modes, see the ESI). Furthermore, we identified 10 additional modes which are the most important to couple the MLCT-3 MC states (with the absolute value of the off-diagonal coupling constant being larger than 0.02 eV, see Figure ); 8 of these have dominant ligand character (out-of-plane twisting, out-of-plane deformation of the middle pyridine ring, C-H stretching), the remaining two are characterised by off-center movement of the central Fe atom and bending modes of the ligand rings.