Partial covalency of hydrogen bonds has long been a topic of debate. 276 For anion-water hydrogen bonds, that debate is settled (in our view) by the observation of strong redshifts in gas-phase vibrational spectra of hydrated anion clusters. These are the signature of n → σ * OH charge donation from the anion into antibonding MOs of the hydrogen-bonded O-H moiety, which is then considerably redshifted whereas the "free" or non-hydrogen-bonded O-H vibration is not. Quantitative values for the fraction of an electron that is transferred vary greatly depending on the theoretical method that is used, but are on the order of a few milli-electrons in the neutral water dimer, 276 up to perhaps ≈ 0.2 electrons for F -(H 2 O). Here, we examine the role of CT in the F -(H 2 O) hydrogen bond using SAPT0 + δHF in conjunction with the cDFT-based procedure for separating polarization and CT (Section 1.3.2). 160 Figure 35(b) plots various energy components as the ion is swept radially around the H 2 O molecule. Note that the C 2v geometry of F -(H 2 O), in which the ion sits at the positive end of water's dipole moment vector, is actually a saddle point between two symmetry-equivalent minima corresponding to quasi-linear hydrogen bonds. This means that halide-water coordination is monodentate rather than bidentate, consistent with the interpretation of cluster ion vibrational spectroscopy, 277-279 solution-phase neutron diffraction, 282 and x-ray absorption experiments. In contrast to the dipolar picture of anion-water interactions that remains common in textbooks, 284 electrostatics alone actually favors the hydrogen-bonded structure over the C 2v geometry. That said, E exch is maximally repulsive in the hydrogen-bonded configuration because the ion-water distance is smaller as compared to that in the C 2v geometry. (Note that the plots in Fig. represent relaxed scans, meaning that the geometry of the complex is optimized at each fixed angle that is used to generate the one-dimensional energy profile. The small cusps that can be seen at the C 2v geometry are artifacts of these relaxed scans that disappear if the H 2 O deformation energy is included, as explained in Ref. 160.) A plot of E int -E CT is nearly flat for angular displacements to either side of the C 2v geometry [Fig. 35(b)], indicating that energy components other than CT are ambivalent towards the position of the ion, so long as it resides within the angle subtended by the two O-H bonds. In contrast, E CT turns on sharply for quasi-linear X -• • • H-O geometries, implying that CT is primarily responsible for stabilizing the hydrogen-bonded configuration relative to the ion-dipole arrangement. Quasi-linearity is a defining feature of the hydrogen bond, and arises from the strong angle dependence of E , CT which can certainly be rationalized in terms of overlap between the MOs n(F) and σ * (O-H), but emerges here from a construction that does not require MO considerations. The same conclusions are valid for complexes X -(H 2 O) with larger halide ions. Encouraged by this successful description, we turned to SAPT(HF) + MBD calculations to analyze anionwater interactions for inorganic anions in aqueous solution. This originates in an attempt to distinguish whether the solvation environment of simple anions (including halides, ClO - x , NO - x , and cyanates) is different in bulk water versus the liquid/vapor interface, and a provocative suggestion that it might not be. Figure shows the results in the form of box-and-whisker plots representing ensemble averages of SAPT(HF) + MBD interaction energies and energy components for anion-water clusters extracted from a simulations. 161 (Each cluster contains approximately two solvation shells of water molecules that are treated as a single monomer.) For a variety of surface-active monovalent anions, we find little difference in E int between the bulk and interfacial environments [Fig. 36(a)]. 161 Perhaps more interestingly, the CT energies are also quite similar [Fig. 36(b)], with the possible exception of I -, ClO -, and SCN -, although even in those cases the differences are modest. Since E int and E CT are similar, it follows that polarization energies are not very difference between bulk and interfacial environments.