Is molecular rectification caused by asymmetric electrode couplings or by a molecular bias drop?

We investigate possible causes of molecular rectification in electrode-molecule-electrode junctions. By using a simple model and simulated conductance histograms, we show that a molecular bias drop is responsible for rectification; conversely, asymmetric molecule-electrode couplings do not directly result in rectification. Instead, the degree of coupling (a)symmetry can be observed in the line shapes of the conductance histograms used to experimentally assess the current-voltage properties of such molecular junctions. More coupling asymmetry leads to less positively skewed histogram peaks.