The ability of the MACE potentials to describe ligand exchange around the metal centre relies on accurately describing the different coordination states and their exchange mechanisms, ideally leading to accurate kinetics. The latter has been difficult to achieve with classical force fields. As discussed previously, ligand exchange in Mg the potential of mean force (PMF) using US with 48 windows (Fig. ). The PMF shows a minimum at 2.1 Å, in agreement with the value obtained from the RDF (Fig. ). A second shallow minimum is located around 4.25 Å, corresponding to the position of the second solvation shell, which is approx. 2 kcal/mol higher than the first minimum. This indicates that the sampling of the region where water leaves the first solvation shell is not fully converged. However, the two repetitions of the US confirm that the free energy barrier is not affected (see Fig. ). The representative structures of both minima and the associated transition region are depicted in Fig. . Analysis of the trajectories confirms that the MACE potential correctly restores the octahedral geometry of the complex. The barrier of pulling the water molecule away from the first solvation shell is 7.6 ± 0.15 kcal/mol, with the peak located at a distance around 3.1 Å. The predicted barrier is 1.9 kcal/mol lower than the experimental value, 9.5 kcal/mol, 4,11 implying a higher rate of water exchange around the Mg 2+ .