  We investigate, by means of first-principles calculations, structural and
transport properties of junctions made of symmetric dithiolated molecules
placed between Au electrodes. As the electrodes are pulled apart, we find that
it becomes energetically favorable that Au atoms migrate to positions between
the electrode surface and thiol terminations, with junction structures
alternating between symmetric and asymmetric. As a result, the calculated
$\emph{IV}$ curves alternate between rectifying and non-rectifying behaviors as
the electrodes are pulled apart, which is consistent with recent experimental
results.
