  Despite its fundamental importance for nano physics and chemistry and
potential device applications, the relationship between atomic structure and
electronic transport in molecular nanostructures is not well understood. Thus
the experimentally observed increase of the conductance of some molecular nano
junctions when they are stretched continues to be counterintuitive and
controversial. Here we explore this phenomenon in propanedithiolate molecules
bridging gold electrodes by means of {\em ab initio} computations and
semi-empirical modeling. We show that in this system it is due to changes in
Au-S-C bond angles and strains in the gold electrodes, rather than to the
previously proposed mechanisms of Au-S bond stretching and an associated energy
shift of the highest occupied molecular orbital and/or Au atomic chain
formation. Our findings indicate that conductance enhancement in response to
the application of tensile stress should be a generic property of molecular
junctions in which the molecule is thiol-bonded in a similar way to gold
electrodes.
