In this study, we report our viewpoint of single molecular conductance in terms of frontier orbitals. The orbital rule derived from orbital phase and amplitude is a powerful guideline for the qualitative understanding of molecular conductance in both theoretical and experimental studies. The essence of the orbital rule is the phase-related quantum interference, and on the basis of this rule a constructive or destructive pathway for electron transport is easily predicted. We have worked on the construction of the orbital rule for more than ten years and recently found from its application that pi-stacked molecular junctions fabricated experimentally are in line with the concept for conductance-decay free junctions. We explain the orbital rule using benzene molecular junctions with the para-, meta-and ortho-connections and discuss linear pi-conjugated chains and p-stacked molecular junctions with respect to their small decay factors in this manuscript.
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