  Inspired by recent measurements of forces and conductances of bipyridine
nano-junctions, we have performed density functional theory calculations of
structure and electron transport in a bipyridine molecule attached between gold
electrodes for seven different contact geometries. The calculations show that
both the bonding force and the conductance are sensitive to the surface
structure, and that both properties are in good agreement with experiment for
contact geometries characterized by intermediate coordination of the metal
atoms corresponding to a stepped surface. The conductance is mediated by the
lowest unoccupied molecular orbital, which can be illustrated by a quantitative
comparison with a one-level model. Implications for the interpretation of the
experimentally determined force and conductance distributions are discussed.
