Several studies examined the water adsorption on oxygen pre-covered Fe(100) surface. At 0.25 ML coverage of oxygen, the water molecule adsorbed on the adjacent iron atom while tilting towards the surface oxygen atom to form the hydrogen bond as shown in Figure (a). However, water adsorption became weaker at the full oxygen monolayer coverage as shown in Figure (b), together with the changes in charge density due to water molecule adsorption. Significant charge density change can be seen at 0.25 ML where the Fe surface atoms transfer charge density towards the surface oxygen atom. The presence of pre-adsorbed oxygen may promote water dissociation to some extent when the surface is partially oxidized. However, when the surface is fully covered with adsorbed oxygen, water dissociation is suppressed because water adsorption is weakened. Figure The side views of the water adsorption on 0.25 ML (a) and 1 ML (b) surface oxygen coverages of Fe(100) and the corresponding charge density difference iso-surfaces. Red and blue regions show electron gain and loss, respectively. (c) The reaction profile of water dissociation on clean and 0.25 ML oxygen-covered Fe(100) surface.