To mimic larger ensembles, we used linear stripes of 3 or 6 Pt atoms per 3 x 3 unit cell, which represent infinitely long strings or double strings of Pt surface atoms along the <110> direction, separated by 1 or 2 strings of Ag surface atoms (Figure , Figure ). These strings were populated with a stepwise increasing number of adsorbed CO molecules. Starting with a single COad per unit cell on a single string of Pt atoms (Figure ), there is little difference in the adsorption energies between on-top adsorption and adsorption on a bridge site. This closely resembles our findings for CO adsorption on a Pt2 dimer, where the difference in adsorption energy (on-top or bridge) was equally small. The adsorption energies are slightly lower than on the Pt2 dimer, in full agreement with a decay in the Ag induced stabilization of the Pt-CO bond discussed in the previous section. Addition of a second COad molecule results in a slightly lower (mean) adsorption energy, independent of whether the two molecules are adsorbed on on-top sites or on bridge sites. Again the effective repulsion are more obvious when using the differential adsorption energy, which decreases from -1.93 eV (-1.95 eV for bridge bonded COad) to -1.83 eV (-1.79 eV for bridge bonded COad). Here we notice also a slight outwards tilt of the COad molecules along the <110> direction to reduce the mutual repulsions. Finally, adding a third COad results in a more significant decrease of the (mean) adsorption energy to -1.67 eV (-1.66 eV), independent of the adsorption site, which corresponds to a rather strong decrease of the differential adsorption energy from -1.83 eV (-1.79 eV) to -1.25 eV (-1.24 eV). In addition to electronic effects, this decrease in adsorption energy is partly caused by fact that the COad molecules can no longer reduce direct repulsions by tilting away from each other along the string direction. Instead, repulsions could only be reduced only by zig-zag type outwards tilts, away from the Pt string, as observed, e.g., in the 2 x 1 structures of CO on Ni(110)