Mg 2+ complex in a water cluster The initial structures for the training were generated using classical MD simulations including a Mg 2+ ion solvated by 325 water molecules in a box of 21.5 Å. The box was minimized and equilibrated to experimental water density using the same procedure as pure water. The system was modelled using the TIP4P-Ew water model combined with Li/Merz ion parameters in TIP4P-Ew water (12-6 HFE set). 10 structures were randomly extracted from the equilibration trajectory and cut into a 7 Å sphere with a centre in the Mg 2+ cation.