  Recently Smit et al. [Nature 419, 906 (2002)] have reported that a single
hydrogen molecule can form a bridge between Pt electrodes, which has a
conductance close to one quantum unit, carried by a single channel. We present
density functional calculations explaining these experimental findings. We show
that the symmetry of the molecular orbitals selects a single conduction
channel. The transmission of this channel is close to unity due to a
combination of the charge transfer between hydrogen and the Pt contacts and the
strong hybridization between the bonding state of the molecule and the d-band
of the Pt leads.
