Returning exclusively to the monovalent ions and examining the other energy components whose statistics are summarized in Fig. , another curiosity arises in regard to dispersion energies for the chlorine oxyanions. Dispersion is size-extensive, so that all else being equal it should scale in proportion to the number of electrons. For the ClO - n species, however, we observe that |E disp | decreases in the order ClO - 3 > ClO - 2 > ClO -> ClO - 4 . This time, perchlorate is the apparent anomaly. Dispersion energies in Fig. ) were computed using the MBD model , so as a sanity check we recomputed E disp using the third-generation ab initio dispersion potential aiD3 , which consists of atom-atom C 6 and C 8 potentials fitted to dispersion-only data from high-level SAPT calculations. Dispersion energies obtain for the ClO - n species with both dispersion models are provided in Table as ensemble averages. Both models afford rather similar dispersion energies, consistent with previous tests for cases where many-body effects on E disp are not significant . (In the context of dispersion, "many-body" implies an effect that cannot be described by pairwise atom-atom potentials . These typically arise in conjugated molecules where screening effects significantly modify the effective C 6 coefficients . For small molecules, three-body dispersion effects are quite small .) Notably, in the aiD3 model the C 6 and C 8 coefficients depend only on atomic number and do not respond to the electronic structure of the monomers.