The third question can be stated as "What is the mechanism of the solvent response and the breaking and forming of hydrogen bonds?". Here, we are primarily interested in a statistically meaningful answer about where each solvent molecule is located at t = 0 and where it is located at the end of the simulation time. In Section S2.6 (Figure and Table ), we have performed such an analysis, by counting the number of water molecules starting and finishing in several volume regions. The most relevant regions are the first solvation shells of the axial and equatorial cyanides and of the bipyridyl ligand (3 regions), the second solvation shells of the cyanides and the bipyridyl (2 regions), and the bulk (see Figure ). Table provides the corresponding correlation matrix after the full 5000 fs simulation time. Here, the most important entries correspond to the amount of water molecules originating and ending in the first solvation shell. Note that the number of water molecules in each region does not change strongly due to the incompressibility of water. Nonetheless, we find that the first solvation shell loses approximately one water and the second solvation shell gains about that much, which is due to the general receding of the water from the excited solvent and also from the fact that the solute expands slightly in the 3 MC states. As illustrated in Fig. , the analysis shows that waters lost from equatorial cyanides move to the cyanide second solvation shell (in the case of 3 MC states), whereas waters lost from axial cyanides end up coordinating the bipyridyl (in the case of 3 MLCT states). In contrast, no directed exchange of waters between the first solvation shell and the bulk is observed. This shows that hydrogen bond reorganization in [Fe(CN) 4 (bipy)] 2-is directed-the water molecules detached from the axial cyanides and the water molecules attached to the bipyridine ligand are in fact the same molecules that migrate along the surface of the solute cavity to form the new hydrogen bonds. This direct mechanism is in contrast to a (more trivial) bulk exchange mechanism, where detached water molecules leave the molecule and other water molecules from the bulk form the new hydrogen bonds.