Potential energy surface plot of CO on Pt(111) surface. Adsorption energy difference is higher at darker places. The positions of CO on top, on bridge, on hcp and on fcc sites are marked "t", "b", "h" and "f", respectively. The solid line shows one of the possible CO diffusion paths.

In the absence of an external electric field, the CO adsorbs on Pt(111) with lowest energy at the fcc site, and the highest energy at the top site, in agreement with recent highly converged calculations. When an external electric field is applied on this system, the binding energy will be changed. Lower coordinate sites become favorable at positive fields. At a critical applied E-field the most favorable site of CO on Pt(111) will switch to the top site. At a field strength of about 0.85 V/Angstrom, the difference in binding energy between the fcc and the top sites reaches zero. The top site becomes the favored site at higher external E-field. The binding energy for the bridge site lies between that of the top and the fcc sites, with and without the external E-field. Both the dipole-field interaction and the geometric effect due to the field energy contribute to the adsorption site preference, with the former being the dominant effect. No significant bonding changes are found. The fact that the potential energy surface is field dependent means that surface diffusion can be affected or may even be controlled by applying an external E-field.