The fact that perchlorate (ClO - 4 ) is an outlier is easy to rationalize in terms of its tetrahedral symmetry and vanishing dipole moment, but the trend amongst the other three chlorine oxyanions is more puzzling. Ensemble-averaged SAPT0 + MBD energy components for the four species ClO - n (aq) are listed in Table , and it is seen that E int , E elst , and E ind all follow the same trend exhibited by the gas-phase dipole moments of the ions in question. However, this means that the trend amongst total interaction energies is different from that predicted by AMOEBA. Instead, for the quantum-mechanical calculations the trend (from strongly to weakly interacting) is