We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density-functional theory ͑DFT͒ as implemented in the well tested SIESTA approach ͑which uses nonlocal norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states͒. We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias ͑including self-consistency and the solution of the electrostatic problem͒ is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: ͑i͒ single atom carbon wires connected to aluminum electrodes with extended or finite cross section, ͑ii͒ single atom gold wires, and finally ͑iii͒ large carbon nanotube systems with point defects.

We report on a self-consistent ab initio technique for modeling quantum transport properties of atomic and molecular scale nanoelectronic devices under external bias potentials. The technique is based on density functional theory using norm conserving nonlocal pseudopotentials to define the atomic core and nonequilibrium Green's functions ͑NEGF's͒ to calculate the charge distribution. The modeling of an open device system is reduced to a calculation defined on a finite region of space using a screening approximation. The interaction between the device scattering region and the electrodes is accounted for by self-energies within the NEGF formalism. Our technique overcomes several difficulties of doing first principles modeling of open molecular quantum coherent conductors. We apply this technique to investigate single wall carbon nanotubes in contact with an Al metallic electrode. We have studied the current-voltage characteristics of the nanotube-metal interface from first principles. Our results suggest that there are two transmission eigenvectors contributing to the ballistic conductance of the interface, with a total conductance GϷG 0 where G 0 ϭ2e 2 /h is the conductance quanta. This is about half of the expected value for infinite perfect metallic nanotubes.

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We present state-of-the-art first principles calculations for the IV characteristics of a donor-insulator-acceptor (DsigmaA) type molecular diode anchored with thiolate bonds to two gold electrodes. We find very poor diode characteristics of the device, and the origin of this is analyzed in terms of the bias-dependent electronic structure. At zero bias, the highest occupied molecular orbital (HOMO) is confined to the D part, and the lowest unoccupied molecular orbital (LUMO) is confined to the A part, while at 3.8 V the two states align, and this gives rise to an increasing current. The latter is a potential mechanism for rectification and may in some cases lead to favorable diode characteristics. We identify the origin of the vanishing rectification for the investigated molecule, and on the basis of this we suggest parameters which are important for successful chemical engineering of DsigmaA rectifiers.