where Finally, we obtain the free energy profiles for O 2 * adsorption and dissociation (Figures and) below) through umbrella sampling simulations. Following previous work, we place harmonic bias potentials at predefined locations ("windows") along the chosen reaction coordinates. These coordinates are chosen here as the O 2 COM height above Pt(111) in the case of adsorption, and a coordination-number-based variable in the case of O 2 * dissociation (cf. SI, Section 3). In both cases, a metadynamics simulation is first performed (starting from an equilibrated image from the unbiased AIMD) so that the entire range of interest along the given reaction coordinate is visited at least once. Initial conditions for each umbrella sampling window are then selected from this trajectory and all windows are sampled for 5 ps. We use the average value of the reaction coordinate in each window to estimate the mean force and obtain the corresponding free energy profile. The resulting energy curves are derived from a single spin-polarized trajectory (per surface charge σ) in the case of O 2 adsorption (Fig. ) and the average over four trajectories (per OH* coverage) in the case of O 2 * dissociation (Fig. )). We further note that we specifically correct for the non-rectilinear nature of the reaction coordinate in the latter case and then integrate over the reactant range with the appropriate Boltzmann weight in order to arrive at the activation free energies shown in Table . Further information on sampling as well as the corresponding analyses of output data is provided in the SI, Section S3. vs Pt(111) interfaces. In this comparison, lower adsorption energy (G ads ) signifies stronger surface binding according to Eq. ( ). All energies are predicted in the same way from AIMD while providing a full atomistic account of the H 2 O solvent, as described in the Methods section above. The metal/liquid interface is explicitly charged in order to model the electrical double layer and the resulting potential drop that drives the catalytic ORR (from left to right along the x-axes of Figure ). We specifically plot here G ads as a function of surface charge density σ (bottom x-axis) within a potential range that is relevant to ORR operation: for Au(111) this includes the experimental reaction onset around 0 V vs SHE 40 or, equivalently, -0.5 V vs the experimental PZC 37 at ca. σ=-12.5 µC/cm 2 according to Eq. (1) (marked by the shaded red region). This value is independent of pH. At Pt(111), however, the experimental ORR onset is well-known to show a Nernstian shift with pH on the SHE scale. This dependence means that the two parameters (U SHE and pH) cannot be decoupled, suggesting a PCET as the rate-determining step. Here, we choose to focus on potential conditions relevant to alkaline ORR and specifically highlight the expected onset value for pH=14 around 0.1 V vs SHE 41 or, equivalently, -0.2 V vs the experimental PZC at ca. σ=-5.0 µC/cm 2 (shaded purple region). As discussed in the Methods section above, however, we emphasize here again that the conversion between σ and U SHE (top and bottom