  The vibrational properties of carbon monoxide adsorbed to the copper (100)
surface are explored within density functional theory. Atoms of the substrate
and adsorbate are treated on an equal footing in order to examine the effect of
substrate--adsorbate coupling. This coupling is found to have a significant
effect on the vibrational modes, particularly the in-plane frustrated
translation, which mixes strongly with substrate phonons and broadens into a
resonance. The predicted lifetime due to this harmonic decay mechanism is in
excellent quantitative agreement with experiment.
