A second interesting observation is the generally strong correlation between classical (AMOEBA) and quantum-mechanical (SAPT) values of E int , even if the former are systematically smaller than the latter, e.g., by 15-19 kcal/mol for the halide ions. (These systematic differences are smaller for the other ions except in the case of ClO - 3 , which is discussed below.) For the halide ions, we use AMOEBA parameters that were originally developed by Ponder and co-workers , and we note that the discrepancies between the force field and the quantum chemistry that are documented in Fig. are much larger than b These data are plotted in Fig. . c These data are plotted in Fig. . those reported in Ref. for binary X -(H 2 O) complexes. This underscores the importance of considering larger ion-water clusters, given the many-body nature of polarization in aqueous systems . Simulation of the hydration free energy of Cl -using AMOEBA results in an error of 11.9 kcal/mol with respect to experiment , assuming that the reference value is defined using the proton solvation energy of Tissandier et al. , which has since emerged as the consensus value . In view of this, the systematic difference of 17 kcal/mol between AMOEBA and SAPT0 + MBD values of E int in bulk water (see Table ) is not so dissimilar from previous results. Improvements to the AMOEBA force field for ions, using SAPT energy components as benchmark data, is a topic of contemporary interest .