The adsorption energy of CO on Co(0001) in Figure also shows a strong repulsive interaction at coverages larger than 4/9 ML. In contrast to Pt(111), CO exhibits attractive interactions at low coverages and reaches a minimum in the binding energy at 1/3 ML, where CO binds more strongly by 3 kJ mol -1 . At 1/3 ML, CO forms a stable ( √ 3 × √ 3)R30 • structure, which has been observed in other theoretical studies and experiments on single crystals . From a Natural Bond Order analysis, Gunasooriya et al. concluded that a reduction of electron density in the anti-bonding Co-C orbitals at 1/3 ML leads to the increase in binding energy. The calculated adsorption enthalpy at 1/3 ML is 130 kJ mol -1 , in good agreement with the experimental value of 128 kJ mol -1 .