  CO adsorption on an Au monatomic chain is studied within density functional
theory in nanocontact geometries as a function of the contact stretching. We
compare the bridge and atop adsorption sites of CO, finding that the bridge
site is energetically favored at all strains studied here. Atop adsorption
gives rise to an almost complete suppression of the ballistic conductance of
the nanocontact, while adsorption at the bridge site results in a conductance
value close to 0.6 G0, in agreement with previous experimental data. We show
that only the bridge site can qualitatively account for the evolution of the
conductance as a function of the contact stretching observed in the
experimental conductance traces. The numerical discrepancy between the
theoretical and experimental conductance slopes is rationalized through a
simple model for the elastic response of the metallic leads. We also verify
that our conductance values are not affected by the specific choice of the
nanocontact geometry by comparing two different atomistic models for the tips.
