  Molecular nanowires in which a single molecule bonds chemically to two metal
electrodes and forms a stable electrically conducting bridge between them have
been studied intensively for more than a decade. However the experimental
determination of the bonding geometry between the molecule and electrodes has
remained elusive. Here we demonstrate by means of ab initio calculations that
inelastic tunneling spectroscopy (IETS) can determine these geometries. We
identify the bonding geometries at the gold-sulfur interfaces of
propanedithiolate molecules bridging gold electrodes that give rise to the
specific IETS signatures that were observed in recent experiments.
