All the anion dimer complexes used in Figure are assumed to be in their doublet states (one unpaired electron), except for the (O 2 + O 2 ) -complexes that are assumed to be in their quartet states (three unpaired electrons). This makes sense as one of the two interacting O 2 monomers will be in its triplet state for very large interatomic separations, while the other monomer is, in its ground state, a doublet. While not presented, doublet state calculations for the (O 2 + O 2 ) - complexes yield similar results, albeit with results deviating slightly more from a linear regression. The geometric configurations used for the anion complexes are the same as the ones used for the neutral complexes. For every panel of Figure , the respective colinear arrangement (θ = 0 for the dimer configurations shown in Figures S2 through S5, in the supplementary material) where the interatomic separation is at its maximum (5 Å) is used as the zeroth energy reference point for the interaction energy calculations.