The most extreme example of heterogeneity is to set ε nonp = 1, for which results are provided in Table . Here, the mean absolute difference between the HetPCM and Voronoi-PEqS values of ∆G elst is 11.2 kcal/mol (or 64% of ∆G elst ) with a maximum deviation of 35.2 kcal/mol. This is clearly an unacceptably large deviation, reflecting the ad hoc nature of the HetPCM construction. It is notable, however, that the largest errors are incurred for the least realistic solvation environments. For example, in the case of NO - 3 there is a 56% difference between HetPCM and Voronoi-PEqS values of ∆G elst when one N-O moiety is in contact with the solvent (represented by ε solv = 78.4), which is reduced to 19% error when the high-dielectric environment surrounds two N-O moieties. Similar remarks can be made for the other ions, for which scheme 1 is a less realistic solvation model because it puts less ion's vdW surface in contact with ε solv . In contrast, scheme 2 surrounds more of the vdW surface with ε solv , which is closer to a homogeneous solvation model.