We next turn our attention to the formation of H-bonds with the solvent and how these stabilize O 2 * under varying potential conditions. As already indicated above, this stabilizing contribution increases at Au(111) around the ORR onset with the transition from a superoxolike to a peroxo-like O 2 * state at the surface. The effect is clearly seen in Figure . We initially count an average number of around 1.5 H-bonds around the PZC (Figure )) when )), magnetic moment (SI, Figure )) and O-O* stretching frequency. (Figure )), ≈-0.8 e Bader charge (SI, Figure )), and almost fully quenched spin of µ ≈0.1-0.2 (SI, Figure )). In this configuration, the molecule lies flat on the surface with its COM ca. 2.0 Å above a bridge site (SI, Figure ). The bridge site preference agrees with our vacuum-background calculations (and those of others ) , but is generally found to vary in the existing literature with sensitive dependence on the exact set-up and chemical environment. At least within our present set-up, including also the (atomistic) H 2 O solvent, our assignment of a dominant peroxo-like O 2 * in bridge is supported by computing the free energy profile of O 2 over Pt(111). Figure Having identified the H-bond donors in the first interfacial H 2 O layer, we now rationalize Figure ) by simply considering the orientation of that H 2 O with respect to the O 2 * adsorbate. In agreement with existing literature, we find that the interfacial H 2 O flips from a rather unstructured O-down configuration at PZC to a much more rigid one-H-down orientation under reducing conditions. This flip is seen in the averaged H(H 2 O) density, shown in the inset panels a) and b) of Figure , as a transition from a single broader peak to two distinct shoulder peaks with increasingly negative σ (cf. also the angle between the H 2 O bisector and surface normal in SI, Figures ). This potential-dependent H 2 O reorientation is similar for the clean Au(111) and Pt(111) interfaces. We note a a similar flip at the OH*-covered Pt(111), but towards a two-H-down (rather than one-H-down) H 2 O configuration (cf. also SI, Fig. ). The O 2 *-H 2 O coordination between Au(111) and Pt(111), however, is quite different. In the case of Au(111), the peroxo-like O 2 * moves away from the surface (COM z-coordinate is marked gray in Figure ) and the H(H 2 O) re-orients towards the surface at increasingly reducing potentials. As a result, the minimum O 2 *-H(H 2 O) distance drops by ca. 0.4-0.5 Å and this gives rise to the potential-dependent increase in the number of H-bonds plotted in Figure ). On the other hand, potential is practically ), however, we count no dissociation around the PZC but only at the more negative σ <-13 µC/cm 2 values (SI,Table and Figure ). We trace this effect back to the potential-dependent change in Pt(111) coverage and resulting availability of free surface sites. Adsorbed H 2 O* is removed from the Pt(111) surface under increasingly reducing conditions, relevant to ORR operation. This prediction agrees perfectly with experiments measuring a potential-dependent preference for non-hydrated over hydrated OH* species at the surface during the ORR. The effect is clearly seen in Figure ) and 4c) (through the disappearing O(H 2 O) peak at z <2.65 Å) and specifically quantified in Figure ), which shows the average surface coverage from AIMD simulations at the clean and OH*-covered Pt(111) models as a function of σ. From these data, we firstly note that the OH* population (empty markers) remains constant for both catalyst models throughout the investigated potential range at 0 ML and 5/12=0.42 ML, respectively (cf. comment in the Methods section above).