Knorr and co-workers have further explored adsorbateadsorbate FOs-mediated interactions. In their experiment, two Co atoms on Ag(111) surface at short distances face strong repelling, so the first potential energy minimum is located at a distance of 27 Å. Unlike the Cu/Cu(111) and Co/Cu(111) systems, the repelling of adsorbates on Ag(111) surface is much stronger, which is evident from the much smaller first potential energy minimum adsorbate-adsorbate distances for the Cu(111) systems (approx. 11 Å) .