To obtain more detail about the local solvation environment, we computed the probability distribution for the number of hydrogen bonds (n HB ) in the first solvation shell of the radical (Fig. ), using standard geometrydependent cutoff parameters. The average value is n HB ≈ 2, including both the hydrogen bonds in which OH is the donor moiety and also those in which it is the acceptor; this represents under-coordination relative to the canonical hydrogen-bonded structure that is shown in Fig. . This soft first-shell structure is in agreement with what we previously obtained from QM/MM simulations using the LRC-ωPBE + D3 functional, as well as from periodic simulations using self-interaction-corrected functionals. Because of the apparent overlap between features in g(r O * O ) that represent hydrogen-bonded versus hemibonded configurations of OH(aq), we turn to an electronic definition of the hemibond based upon spin delocalization (or "spin leakage") from the radical to the first-shell water molecules. We have previously used such a metric to infer the existence of hemibonded configurations in QM/MM simulations, 39 by defining the "spin charge" s OH to be the sum of Mulliken charges on the radical, computed using the spin density ρ αρ β . The value of s OH can distinguish hemibonding from hydrogen bonding even if the former is a rare event, because the hemibond owes its existence to favorable overlap between the half-occupied O * (2p) orbital and a lone pair orbital of a nearby water molecule, leading to spin leakage upon formation of the 2c-3e bond.