  CO adsorption on Cu(111) and Cu(001) surfaces has been studied within
ab-initio density functional theory (DFT). The structural, vibrational and
thermodynamic properties of the adsorbate-substrate complex have been
calculated. Calculations within the generalized gradient approximation (GGA)
predict adsorption in the threefold hollow on Cu(111) and in the bridge-site on
Cu(001), instead of on-top as found experimentally. It is demonstrated that the
correct site preference is achieved if the underestimation of the HOMO-LUMO gap
of CO characteristic for DFT is correct by applying a molecular DFT+U approach.
The DFT+U approach also produces good agreement with the experimentally
measured adsorption energies, while introducing only small changes in the
calculated geometrical and vibrational properties further improving agreement
with experiment which is fair already at the GGA level.
