  Recent experiments showed that the conductance of Pt nanocontacts and
nanowires is measurably reduced by adsorption of CO. We present DFT
calculations of the electronic structure and ballistic conductance of a Pt
monoatomic chain and a CO molecule adsorbed in an on-top position. We find that
the main electronic molecule-chain interaction occurs via the $5\sigma$ and
$2\pi^{\star}$ orbitals of the molecule, involved in a donation/back-donation
process similar to that of CO on transition-metal surfaces. The ideal ballistic
conductance of the monoatomic chain undergoes a moderate reduction by about 1.0
G_0 (from 4 G_0 to 3.1 G_0) upon adsorption of CO. By repeating all
calculations with and without spin-orbit coupling, no substantial spin-orbit
induced change emerges either in the chain-molecule interaction mechanism or in
the conductance.
