The oxygen at hollow sites behaves very differently from that at the bridge sites. The Ea on the hollow sites (red) reduces (less negative) rapidly, hits the least negative turning point (Ea ≈ -2 eV) at approximately U ≈ -1.13 V/SHE and increases (more negative) as the potential decreases (less negative). Figure reveals that if the potential is outside of the vicinity of -1.20 V/SHE, either more negative or less negative potentials lead to a stabilization of the oxygen adsorption on the Fe(100) surface. This behaviour likely arises from the multicoordination nature of oxygen at hollow sites, where it interacts with several Fe surface atoms.