The sharp drop in dispersion between chlorate (ClO - 3 ) and perchlorate is a feature of both dispersion models, suggesting that this is not an artifact. A likely explanation is that in perchlorate, the addition of a fourth oxygen atom around the central (and more polarizable) chlorine atom screens the water molecules from this polarizable center, and thus significantly attenuates chlorine's contribution to the dispersion energy. In contrast, for the other ClO - n ions the chlorine atom remains solvent-exposed and the dispersion is much larger. This mechanism would be reflected in both dispersion models, if only as a function of increased chlorine-water distance in the aiD3 case. Also in support of this hypothesis are the data in Fig. (b) for SAPT0 + MBD induction energies, which also exhibit a pronounced drop in magnitude between ClO - 3 and ClO - 4 . As compared to dispersion interactions, polarization effects decay more slowly with distance, e.g., as r -4 for charge-dipole polarization, but this dependence is still rather steep. Polarization is often invoked in discussions of ions at the air/water interface , so it is interesting to note that induction energies are systematically smaller in the interfacial environment [Fig. ]. As with the total interaction energies, however, the difference between bulk and interfacial mean values E ind is small in comparison to the instantaneous fluctuations as measured by the standard deviation. The numerical data corresponding to Fig. (b) are provided in Table . Note that "polarization" as it is typically understood means strictly intramolecular redistribution of charge, with CT considered as a separate effect, and these two parts of the induction energy are separated in Fig. . Because the CT-free polarization energy (E pol ) is much larger than the CT energy (E CT ), the result is that E pol follows essentially the same trend from ion to ion as does the total induction energy, E ind . In particular, this means that the polarization energy is systematically smaller in the interfacial environment, for each of the ions considered here. Indeed, for the canonical surface-active anions Br -, I -, ClO - 4 and SCN - , the polarization energy is significantly smaller in the interfacial environment, by at least the standard deviation of E pol in bulk water; see Fig. .