Green function and first principle theories are used to investigate electron transport through the polyphenylene-based molecular rectifying diode switch, which is attached to gold electrodes at both ends. The coupling at the interface between the molecular device and the electrodes is treated carefully. The Green function of the electrode is built by the standard parameterized tight-binding method. The effect of the molecular orbitals on the conductance is shown clearly in the conductance curve. The conductance calculated from the approximated Green function method, which is composed of spd/sp/s model, is presented. The difference between our Green function and the current approximated Green function is discussed. The result suggests that the Green function made from sp orbital model is a good approximation. Besides, the molecular structure of the diode-diode-type AND gate, composed of the polyphenylene-based molecular wires, is energetically optimized. Some of its orbitals, near the Fermi level, are presented. Our calculation method can be extended straightforwardly to any organic molecular system, which is connected by the electrodes.
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