Figure illustrates the highly variable bias dependence of Φmax for specific metal surfaces. For example, Mo(110) is an inactive catalyst at 0 V because of its large endergonic barrier (Φmax = 1.58 eV) to protonate NHNH2* three times to yield two NH3 molecules. However, because three electron transfers are required to transform NHNH2* to two NH3 molecules, the energetic span on Mo(110) is highly sensitive to bias. This leads to a substantial decrease in Φmax from 1.58 eV at 0.0 V to 0.77 eV at -0.25 V, making Mo(110) the third most active metal for catalyzing NRR at -0.25 V as predicted by Φmax. However, at -0.5 V Mo(110) becomes limited by the chemical step of NH3* desorption and the energetic span of Mo(110) becomes minimally sensitive to bias (Φmax = 0.57 eV at -0.5 V). Mo(110) is predicted to behave nonlinearly with bias because it proceeds through the associative alternating pathway, which results in an energy trough at the dissociated NHNH2* state. In contrast, Ru(211) is minimally sensitive to changes in bias (Φmax = 0.96 and 0.84 eV at 0.0 and -0.5 V, respectively) because its energetic span is always limited by the chemical step of NH3* desorption. Consequently, Ru(211) is calculated to have the lowest Φmax of the studied surfaces at 0.0 V, but only the 8 th lowest Φmax at -0.5 V. The Φmax of Ru(211) at -0.5 V deviates from previous studies that place it toward the top of the activity volcano, and suggests that Ru(111) is the more active facet on Ru metal at -0.5 V. The differences between Ru(111) and Ru(211) primarily arise from the different number of electron transfers associated with different energetic spans, however the bias dependence of Φmax on Ru(211) is a direct consequence of studying the chemical NRR steps with GC-DFT at each applied bias.