because their sum approximates the electrostatic interaction between antisymmetrized monomer wave functions. This combination of "primitive" electrostatics (E elst , which is the Coulomb interaction between isolated-monomer charge densities) and Pauli repulsion (E exch ) has proven to be easier to interpret for halide-water systems as compared to electrostatics alone . An example can be found in the ensemble-averaged energy components for the ClO - n (aq) species (Table ), where the much less repulsive value of E exch for perchlorate at first seems at odds with the larger size of this ion. However, the reduced Pauli repulsion in this case is actually commensurate with a much less attractive value of E elst , suggesting a hydration sphere that is not as tight around the ion as it is in smaller (but electrostatically much more attractive) ClO - n ions. Statistical distributions of E elst+exch are shown in Fig. (a) for all of the ions, and immediately ClO - 3 stands out as the only ion for which E elst+exch > 0, meaning that the sum of first-order interactions is net repulsive in this case but is net attractive for each of the other ions. These observations are independent of whether one considers the bulk or interfacial data sets because differences between the bulk and interfacial mean values of E elst+exch are tiny in comparison to the instantaneous fluctuations, as was the case for E int . Furthermore, this anomalous prediction regarding ClO - 3 is not unique to the SAPT0 level of -