Figure 10: HS-LS Differences complex-shell (V CS , red), water-water (V WW , blue), and total (V , purple) interaction energies in the [Fe(bpy 17, 18, 19, 20) at the B3LYP-D3(BJ)/def2-TZVP geometries. The fully drawn lines represent the B3LYP-D3(BJ) results, whereas the dashed lines show the OPLS/TIP4PEW results. The main discrepancies are found to arise from the complex-shell interaction energy. from changes in the direct interactions between solute and solvent, but rather in changes in interactions between the water molecules themselves, induced by the geometric and electronic changes within the complex. This, again, increases the challenge for purely pairwise additive models such as OPLS/TIP4PEW, which does not include changes in water-water interactions due to e.g. polarization induced by the electronic density of the complex in its potential. Looking at V WW , and recalling that it can only include polarization effects between the waters themselves, we see that the TIP4PEW model, not including any nonpairwise additive contributions at all, still manage to reproduce this term with a standard deviation from B3LYP-D3(BJ) of only 13.6 meV. However, the complex-shell interactions of B3LYP-D3(BJ) are less accurately captioned by the OPLS/TIP4PEW force field, even though we have seen in the previous sections that the MM model performs relatively well for the smaller dimer-tetramer systems, where many-body effects matter less. Here, the model predicts a qualitatively different response to electronic excitation for 18+ water shells, becoming less binding, whereas the B3LYP-D3(BJ) model becomes more binding. It would be reasonable to expect that the complex, being positively charged, would be the main cause for polarization between the waters. We will analyse this effect in more detail in the next section.