Formalism, analytical model, and a priori Green's-function-based calculations of the current-voltage characteristics of molecular wires

Various Green's-function-based formalisms which express the current I as a function of applied voltage V for an electrode-molecule-electrode assembly are compared and contrasted. The analytical solution for conduction through a Huckel (tight binding) chain molecule is examined and only one of these formalisms is shown to predict the known conductivity of a one-dimensional metallic wire. Also, from this solution we extract the counter-intuitive re sult that the imaginary component of the self-energy produces a shift in th e voltage at which molecular resonances occur, and complete analytical desc riptions are provided of the conductivity through one-atom and two-atom bri dges. A method is presented by which a priori calculations could be perform ed, and this is examined using extended-Huckel calculations for two gold el ectrodes spanned by the dithioquinone dianion. A key feature of this is the use of known bulk-electrode properties to model the electrode surface rath er than the variety of more approximate schemes which are in current use. T hese other schemes are shown to be qualitatively realistic but not sufficie ntly reliable for use in quantitative calculations. We show that in such ca lculations it is very important to obtain accurate estimates of both the mo lecule-electrode coupling strength and the location of the electrode's Ferm i energies with respect to the molecular state energies. (C) 2000 American Institute of Physics. [S0021-9606(00)70303-5].