Figure shows the polarization energy (E pol = E ind -E CT ) that is obtained using either the Becke or the FBH weighting function to define the charge constraint. (Both definitions of E pol start from the same SAPT0 induction energy, E ind .) It is apparent that the two definitions afford step-to-step fluctuations that do not seem to correlate with one another. In the Becke definition, the size and shape of the Voronoi cell that contains the iodide anion is sensitive to the instantaneous values of all iodide-water distances in the first solvation shell, whereas the FBH definition uses a spherically-symmetric charge density for the isolated anion in order to define the charge constraint; the latter definition is less sensitive to fluctuations in the atomic coordinates. (Note that FBH definition certainly remains sensitive to the presence of hydrogen bonds .) CT energies for snapshots of I -(aq) in bulk water, computed using a charge-constrained SCF procedure with the charge constraint defined either using fragment-based Hirshfeld (FBH) weights (scale at left), or else Becke's multicenter partitioning procedure (scale at right). Results using the Becke scheme include the "atomic size adjustments" that are described in Ref. 69, wherein Slater's set of atomic radii are used to adjust the boundaries of the Voronoi cells based on atomic size. For I -(aq), it is consistently the case that the CT-free reference state defined using Becke partition results in CT energies that are larger in magnitude: |E CT (Becke)| > |E CT (FBH)|. This is evident from the rather different energy scales in Fig. , but the situation is not the same for all of the anions. As a second example we consider ClO -(aq), which exhibits the largest values of |E CT | of any of the ions studied here, at least when the FBH definition is used. Figure considers both definitions and examines how E CT fluctuates from snapshot to snapshot. Becke's partition predicts very little CT for ClO -in bulk water ( E CT = -1.2 kcal/mol) whereas the FBH definition results in an average value of E CT = -6.2 kcal/mol. In either case, E CT is consistently larger for the interfacial snapshots.