The conductance of a benzene molecule connected to two ruthenium (Ru) electrodes through two C(H) anchoring groups is investigated using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with density functional theory. Our calculations demonstrate that a nearly perfect conductance with magnitude exceeding 84% of the conductance quantum GO can be obtained when the two C(H) anchoring groups are bonded to a Ru adatom on the Ru(0001) surface, independently from whether this is a Ru@C double bond or a Ru„C triple bond. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the benzene backbone interact with the Ru-C p bonds in the contact region to form efficient charge transport channels, illustrating the high conducting nature of benzene.

An efficient self-consistent approach combining the nonequilibrium Green's function formalismwith density functional theory is developed to calculate electron transport properties of moleculardevices with quasi-one-dimensional (1D) electrodes. Two problems associated with the lowdimensionality of the 1D electrodes, i.e., the nonequilibrium state and the uncertain boundaryconditions for the electrostatic potential, are circumvented by introducing the reflectionlessboundary conditions at the electrode-contact interfaces and the zero electric field boundaryconditions at the electrode-molecule interfaces. Three prototypical systems, respectively, an idealballistic conductor, a high resistance tunnel junction, and a molecular device, are investigated toillustrate the accuracy and efficiency of our approach.

We discuss two problems in the conventional approach for studying charge transport in molecular electronic devices that is based on the non-equilibrium Green's function formalism and density functional theory, i.e., the bound states and the numerical integration of the non-equilibrium density matrix. A scheme of filling the bound states in the bias window and a method of patching the non-equilibrium integration are proposed, both of which are referred to as the non-equilibrium correction. The discussion is illustrated by means of calculations on a model system consisting of a 4,4 bipyridine molecule connected to two semi-infinite gold monatomic chains.

The conductance of a family of ruthenium-octene-ruthenium molecular junctions with different πconjugation are investigated using a fully self-consistent ab initio approach which combines thenonequilibrium Green's function formalism with density functional theory. Our calculationsdemonstrate that the continuity of the π conjugation in the contact region as well as along themolecular backbone affects the junction conductance significantly, showing the advantage of usingthe ruthenium-carbon double bond as the linkage of conjugated organic molecules.

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between twosingle-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistentab initio approach which combines nonequilibrium Green's function formalism with densityfunctional theory calculations. Several metallic zigzag and armchair SWCNTs with differentdiameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogenatoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couplestrongly only with the frontier molecular orbitals that are related to nonlocal π bonds. Although thechirality of SWCNT electrodes has significant influences on this coupling and thus the molecularconductance, the diameter of electrodes, the distance, and the torsion angle between electrodes haveonly minor influences on the conductance, showing the advantage of using SWCNTs as theelectrodes for molecular electronic devices.

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.