The vibrational properties of carbon monoxide adsorbed to the copper (
100) surface are explored within density-functional theory. Atoms of t
he 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, particula
rly the in-plane frustrated translation, which mixes strongly with sub
strate phonons and broadens into a resonance. The predicted lifetime d
ue to this harmonic decay mechanism is in excellent quantitative agree
ment with experiment.