The test set includes ten snapshots each of both of the ions in our MD simulations (Mg 2+ and Cl -) surrounded by 20 water molecules as well as ten structures of 20 water only clusters with no ions (S7 Table ). These test structures are slightly larger than the solvent data the model was trained on, which were water clusters consisting of 15 molecules . Water molecules in the test structures are categorized into one of three water shells based on the distances of their O atoms from the ion (for the water only clusters, because there is no ion we use the distance from the center of nuclear charge for the cluster). Taking the default values from Amber's CPPTRAJ analysis program , the first water shell is 0 < r <= 3.4 Å, the second water shell is 3.4 < r <= 5.0 Å, and the third shell contains the remaining waters r > 5.0 Å. Dipole moments for individual water molecules were calculated by integrating their electron densities on a 0.1 Bohr cubic grid. These electron densities were partitioned by using only the output basis functions for the respective atoms in the water molecule.