WMetaD is performed with the La-Cl interionic distance of the dissociating Cl -as the collective variable (CV). The other Cl -ligands are prevented from dissociating or associating by restraining them to a fixed distance either close to or far away from the La 3+ ion. For example, in case of reaction 2, one Cl -ligand is restrained at ∼3 Å, the other is allowed to associate while the third one (to be associated in the next step) is restrained at ∼8-10 Å. This is illustrated in Figure (left, inset). In dilute solutions ions generally exist in equilibrium as contact ion pairs (CIPs -ion pairs where anion and cation are in close contact), solvent-shared ion pairs (SShIPs -ion pairs where anion and cation share hydration water molecules) and solvent-separated ion pairs (SSIPs -ion pairs where anion and cation form their individual hydration shells) as shown in Figure . These can be Inset 2 shows the dissociated state where one of the previously coordinating Cl -ions is at a long distance. Right: