We present a theoretical approach which allows one to extract the orbital contribution to the conductance of molecular electronic devices. This is achieved by calculating the scattering wave functions after the Hamiltonian matrix of the extended molecule is obtained from a self-consistent calculation that combines the nonequilibrium Green's function formalism with density functional theory employing a finite basis of local atomic orbitals. As an example, the contribution of molecular orbitals to the conductance of a model system consisting of a 4,4-bipyridine molecule connected to two semi-infinite gold monatomic chains is explored, illustrating the capability of our approach.

Two popular models of the gold-4,4 bipyridine (44BPD)-gold molecular junction, i.e., the direct contact of the 44BPD molecule with the Au(111) surface and the intermediary contact through one extra gold atom on each side, were studied using density functional theory calculations under periodic boundary conditions. The relative position of the Fermi level is changed by the extra gold atom from well below the LUMO(lowest unoccupied molecular orbital) of the 44BPD molecule in the direct contact model to within the energy range of theLUMOin the intermediary contact model, indicating that the local structure of the contact can significantly affect the conducting characteristics of the junction. The dependence of the molecule–electrode interaction on the interface structure was also investigated in details.

Three types of 11-electron analytic effective core potentials (ECPs) and their corresponding double-ú and single-ú basis sets of gold are evaluated using density functional theory (DFT) calculations. We find that, compared with basis sets derived for use with Hatree-Fock-based Los Alamos (LANL1) and Ermler- Christiansen (EC) ECPs, the DFT-derived Troullier-Martins (TM) ECP together with a single-ú basis set (TMSZ) is more suitable to describe not only the interaction between gold atoms with a benzene-1,4-dithiolate molecule but also the electronic structure of an infinite 1-dimensional monatomic gold chain. Hence, TMSZ is the best single-ú basis set with an 11-electron ECP for gold available currently to be used in theoretical calculations on electrical properties of molecular electronic devices with DFT based Green's function method employing a finite analytic basis of local orbitals.

Reversible conductance transitions are demonstrated on the molecular scale in a complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine, by application of local electric field pulses. Both macroscopic and local current-voltage (I-V) measurements show similar electrical bistability behavior. The mechanism of the electrical bistability is discussed.