We present a first-principles numerical implementation of Landauer formalism for electrical transport in nanostructures characterized down to the atomic level. The novelty and interest of our method lie essentially on two facts. First of all, it makes use of the versatile GAUSSIAN98 code, which is widely used within the quantum chemistry community. Second, it incorporates the semi-infinite electrodes in a very generic and efficient way by means of Bethe lattices. We name this method the Gaussian embedded cluster method (GECM). In order to make contact with other proposed implementations, we illustrate our technique by calculating the conductance in some well-studied systems such as metallic (Al and Au) nanocontacts and C-atom chains connected to metallic (Al and Au) electrodes. In the case of Al nanocontacts the conductance turns out to be quite dependent on the detailed atomic arrangement. In contrast, the conductance in Au nanocontacts presents quite universal features. In the case of C chains, where the self-consistency guarantees the local charge transfer and the correct alignment of the molecular and electrode levels, we find that the conductance oscillates with the number of atoms in the chain regardless of the type of electrode. However, for short chains and Al electrodes the even-odd periodicity is reversed at equilibrium bond distances.

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNT’s considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNT’s. Al and Au turn out to make poor contacts while Ti is the best option of the three.

The design of advanced N-doped carbon materials towards oxygen reduction reaction (ORR) catalysis is only possible if the nature of the active sites is fully understood. There is an important piece of research seeking to overcome this challenge through experimental or theoretical results. However, the combination of both approaches is necessary to deepen the knowledge about this subject. This work presents excellent agreement between experimental results and computational models, which provides evidence of the nature of the most active sites in N-doped carbon materials. N-doped carbon materials have been experimentally obtained through double stage treatment of polyaniline in distinct atmospheres (both oxygen-containing and inert atmospheres) at different temperatures (800–1200 °C). According to temperature programmed desorption (TPD), Raman spectroscopy, N₂-adsorption isotherms at −196 °C and X-ray photoelectron spectroscopy (XPS), this synthesis method results in the selective formation of nitrogen species, without significant changes in structural order or porosity. ORR catalytic tests evidence the highly efficient catalysis, with platinum-like performance in terms of the current density and onset potential, of N-doped carbon materials selectively containing graphitic-type nitrogen species. Computational chemistry, through DFT calculations, shows that edge-type graphitic nitrogen is more effective towards ORR catalysis than pyridinic, pyrrolic, pyridonic, oxidized and basal-type graphitic nitrogen species.

We describe a new algorithm for the generation of 3D grids for the numerical evaluation of multicenter molecular integrals in density functional theory. First, we use the nuclear weight functions method of Becke [A. D. Becke, J. Chem. Phys. 88, 2547 (1988)] to decompose a multicenter integral ∫F(r) dr into a sum of atomic‐like single‐center integrals. Then, we apply automatic numerical integration techniques to evaluate each of these atomic‐like integrals, so that the total integral is approximated as ∫F(r) dr≊∑_iω_iF(r_i). The set of abscissas r_i and weights ω_i constitutes the 3D grid. The 3D atomic‐like integrals are arranged as three successive monodimensional integrals, each of which is computed according to a recently proposed monodimensional automatic numerical integration scheme which is able to determine how many points are needed to achieve a given accuracy. When this monodimensional algorithm is applied to 3D integration, the 3D grids obtained adapt themselves to the shape of the integrand F(r), and have more points in more difficult regions. The function F(r), which, upon numerical integration, yields the 3D grid, is called the generating function of the grid. We have used promolecule densities as generating functions, and have checked that grids generated from promolecule densities are also accurate for other integrands. Our scheme is very reliable in the sense that, given a relative tolerance ε, it generates 3D grids which are able to approximate multicenter integrals with relative errors smaller than ε for all the molecules tested in this work. Coarser or finer grids can be obtained using greater or smaller tolerances. For a series of 21 molecules, the average number of points per atom for ε=2.0⋅10⁻³, ε=2.0⋅10⁻⁴, ε=2.0⋅10⁻⁵, ε=2.0⋅10⁻⁶, and ε=2.0⋅10⁻⁷ is respectively 3141 (2.9⋅10⁻⁴), 10271 (2.4⋅10⁻⁵), 27184 (3.1⋅10⁻⁶), 72266 (1.9⋅10⁻⁷), and 164944 (5.2⋅10⁻⁹) (in parentheses are the maximum errors obtained when integrating the density). It is possible to reduce the number of points in the grid by taking advantage of molecular symmetry. It seems that our method achieves a given accuracy with fewer points than other recently proposed methods.

Some aspects related to the behavior of the correlation energy as a functional of the density are not well represented by the density functionals; such is the case of the energy curves for pure states of diatomic molecules, the dependence of the correlation energy on the zeroth-order wave function, or the multiplet splitting in atoms. In this paper we show that these limitations are due to the fact that a complete description of the two-body effects requires one to take into account the two-electron density. It is shown that using very simple approximations to incorporate explicitly the two-body density into the density-functional equations improves their behavior considerably.

We discuss the key steps that have to be followed to calculate quantum transport out of equilibrium by means of the ab initio Gaussian embedded-cluster method recently developed by the authors. Our main aim is to emphasize that, if a sufficiently large portion of the electrodes is included in the ab initio calculation, there is no need to impose an electrostatic potential V drop across the system. The electrochemical-potential difference ${\mu }_L-{\mu }_R=eV$ (where ${\mu }_L$ and ${\mu }_R$ are the electrochemical potentials well into the left and right electrodes), which is also incorporated in the method, suffices to induce a charge redistribution that creates an electrostatic drop across the constriction. The discussion is illustrated by means of quantum transport calculations through aluminum and gold nanocontacts.