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Mechanism of the enhanced conductance of a molecular junction under tensile stress

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.