Orientational effects in molecule–surface interactions: bonding directionality versus steric repulsion

The potential energy surface (PES) describing the lateral motion of CO chemisorbed on a Cu(001) surface has been computed from first principles. The vibrational energies for lateral motion obtained with this PES are in excellent agreement with recent experimental data. The isotope shifts have also been computed and provide direct evidence for strong coupling of translation and rotation. In addition the results reveal an unexpectedly large directionality of the CO–substrate interaction, which is not due to chemical bonding effects. Instead the effective corrugation of the surface conduction band charge density is identified as the origin for this large directionality thus illustrating the complexity of the molecule–surface bond.