Figure shows the joint probability distribution in (s OH , r O * O ), where r O * O is the distance from O * to the nearest water oxygen. Values r O * O < 2.5 Å are expected for hemibonded configurations and these smaller values do indeed correlate with spin leakage, as measured by s OH . Based on these joint probability distributions we selected a cutoff value of s OH = 0.97 to define the hemibonded configurations. The distribution in Fig. shows that this electronic definition of the hemibond comports with a geometric definition based on r O * O . Figures and S6 examine one-and two-coordinate probability distributions for the geometric parameters that have previously been used to define the hemibond (r O * O and two angles). One-dimensional probability distributions (Fig. ) are similar to what we obtained in QM/ MM simulations using the LRC-ωPBE functional, and the joint probability distributions (Fig. ) demonstrate that geometric definitions of the hemibond agree with the electronic definition based on s OH . In other words, a hemibond in the traditional sense was present all along in periodic ai MD simulations based on hybrid functionals, but was been overlooked because it does not give rise to the distinct feature in g(r O * O ) that has previously been assumed to be the hallmark of hemibonded OH(aq). From QM/MM simulations using a variety of density functionals, we know that the average O * • • • O distance associated with hemibonding shifts to slightly larger values of r O * O as the fraction of exact exchange is increased (Table ). This is how the hemibonded radical manages to hide beneath the larger hydrogen-bonded feature in the RDF, in ai MD simulations using hybrid functionals.