for reasons that are discussed in Ref. . In the analysis of hydrogen bonding it is often of interest to separate these effects but that separation has historically been considered problematic, and not just within the SAPT formalism; many schemes for separating polarization from CT exhibit strong dependence on the choice of basis set . To accomplish the separation in Eq. ( ) in a robust way that converges rapidly with respect to basis set, we use the machinery of a charge-constrained self-consistent field (SCF) calculation in order to define a CT-free reference state. Here, the monomers are allowed to polarize one another but their charge densities are constrained to integrate to integer numbers of electrons. Because the SCF procedure is variational, lifting of this constraint necessarily lowers the energy (to that of the fully-relaxed SCF solution), and this energy lowering is taken to define E CT . The CT energy thus obtained is then subtracted from the induction energy to obtain the CT-free polarization energy, E pol = E ind -E CT . CT energies defined in this way are very nearly converged already in double-ζ basis sets . This approach has previously been used to demonstrate that E CT furnishes a driving force for formation of quasi-linear hydrogen bonds in binary halide-water complexes . Implementation of the charge-constrained SCF procedure requires a method to count electrons, and Becke's multicenter partition scheme is commonly used for this purpose . This approach divides space into Voronoi cells , which are regions of space that are closest to a particular nucleus, and then Becke applies a smoothing function at the boundaries of these polyhedra. Alternatively, and specifically for the purpose of defining a CT-free reference state in order to effect the partition suggested in Eq. ( ), a counting procedure based on fragment-based Hirshfeld (FBH) weighting has also been suggested . In the latter approach, the number of electrons contained in fragment A is defined as