Water cluster subset The initial structures of the bulk water system were prepared by classical MD simulation using TIP4P-Ew FF. A cubic box (L= 15 Å) was solvated with 112 water molecules, minimized and equilibrated in an NPT ensemble (300 K and 1.0 bar) for 1 ns using Langevin dynamics and Berendsen barostat as implemented in the sander module of Ambertools23. For the initial training set, 10 clusters of 7 Å radius containing 45 to 49 water molecules were cut from the equilibrated trajectory and labelled with the reference energies and forces (vide infra). The training set was then enhanced by AL using the similarity selector with a threshold of 0.9998 to avoid selection of too distorted structures, and a maximum time set to 10 ps to accommodate for water relaxation. This approach yielded an overall 149 structures.