  We have studied the vibrational properties of CO adsorbed on platinum and
platinum-ruthenium surfaces using density-functional perturbation theory within
the Perdew-Burke-Ernzerhof generalized-gradient approximation. The calculated
C-O stretching frequencies are found to be in excellent agreement with
spectroscopic measurements. The frequency shifts that take place when the
surface is covered with ruthenium monolayers are also correctly predicted. This
agreement for both shifts and absolute vibrational frequencies is made more
remarkable by the frequent failure of local and semilocal exchange-correlation
functionals in predicting the stability of the different adsorption sites for
CO on transition metal surfaces. We have investigated the chemical origin of
the C-O frequency shifts introducing an orbital-resolved analysis of the force
and frequency density of states, and assessed the effect of donation and
backdonation on the CO vibrational frequency using a GGA + molecular U
approach. These findings rationalize and establish the accuracy of
density-functional calculations in predicting absolute vibrational frequencies,
notwithstanding the failure in determining relative adsorption energies, in the
strong chemisorption regime.
