Constructive and destructive interference are typical features of electron transport in molecular junctions, which appear as parabolic curves and sharp dips of transmission functions, respectively. To understand the quantum interference properties in molecular junctions, the Green's function method with tight-binding models was adopted, and the quantum interference was analyzed in terms of orbitals, which leads to an efficient orbital rule for qualitative predictions of electron transport in molecular junctions. A minimummodel, a two-site tight-binding model, was used to explain the orbital rule for electron transport without ambiguity. The orbital bases in tight-binding models are typically atomic orbitals, and thus the tight-binding model can be easily extended to larger molecules by simply adding atomic sites. As the next example, a three-site triangular tight-binding model was introduced. The quantum interference that appears in the three-site model can be easily understood using the orbital rule. With regard to the orbital bases as molecular orbitals, the triangular tight-binding model could efficiently explain the destructive interference recently observed in a large molecular unit. In the final part, we also examine the applicability of the orbital rule for molecular spin systems including spin-flip processes.
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