  We use different semi-empirical dispersion correction schemes to assess the
role of long-range van der Waals interactions in the adsorption of the
prototypical molecular switch azobenzene (C6H5-N2-C6H5) at the coinage metal
surfaces Cu(111), Ag(111) and Au(111). Compared to preceding density-functional
theory results employing a semi-local exchange and correlation functional we
obtain partly sizable changes of the computed adsorption geometry and
energetics. The discomforting scatter in the results provided by the different
schemes is largely attributed to the unknown form of the damping function in
the semi-empirical correction expression. Using the congeneric problem of the
adsorption of benzene as a vehicle to connection with experiment, we cautiously
conclude that the account of dispersive interactions at the metal surfaces
provided by the various schemes is in the right ballpark, with the more recent,
general schemes likely to overbind.
