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 lies essentially on two facts. First of all, it makes use of the versatile Gaussian98 code, which is widely used within the quantum chemistry community. Secondly, it incorporates the semiinfinite 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 wellstudied 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. On the contrary, 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 have investigated the origin of the S1‐T1 energy levels inversion for heptazine, and other N‐doped π‐conjugated hydrocarbons, leading thus to an unusually negative singlet‐triplet energy gap (ΔEST<0 ). Since this inversion might rely on substantial doubly‐excited configurations to the S1 and/or T1 wavefunctions, we have systematically applied multi‐configurational SA‐CASSCF and SC‐NEVPT2 methods, SCS‐corrected CC2 and ADC(2) approaches, and linear‐response TD‐DFT, to analyze if the latter method could also face this challenging issue. We have also extended the study to B‐doped π‐conjugated systems, to see the effect of chemical composition on the results. For all the systems studied, an intricate interplay between the singlet‐triplet exchange interaction, the influence of doubly‐excited configurations, and the impact of dynamic correlation effects, serves to explain the ΔEST<0 values found for most of the compounds, which is not predicted by TD‐DFT.

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 into the knowledge about this subject. This work presents excellent agreement between experimental results and computational models, which provides evidence about 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 atmosphere) at different temperatures (800-1200ºC). According to temperature programmed desorption (TPD), Raman spectroscopy, N 2 -adsorption isotherms at -196ºC and Xray photoelectron spectroscopy (XPS), this synthesis method provides the selective formation of nitrogen species, without important changes in structural order or porosity. ORR catalytic tests evidence the highly efficient catalysis, with platinum-like performance in 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.After the electron supply, the proton must be introduced into the model structure to complete the first reduction stage. Fig. 7a illustrates this process. The proton is attracted towards the oxygen atoms that have collected the electrons forming an intramolecular hydrogen bond.

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. Additional information on the CNT band mixing at the contacts is also obtained.Controversy on the observed electrical transport properties of carbon nanotubes (CNT's) has been mostly due to our lack of control and understanding of their contact to the metallic electrodes. It has finally become clear that the contact influences critically the overall performance of the CNT and that it is crucial to lower the inherent contact resistance to achieve the definite understanding of the intrinsic electrical properties of CNT's [1,2,3]. In order to determine the relevant factors behind the contact resistance so that this can be pushed down to its alleged quantum limit R 0 = h/2e 2 per CNT channel a big experimental effort has been made both in CNT growth and lithographic techniques [4,5,6,7,8,9,10]. While considerable progress in this direction has already been achieved, theoretical progress, on the other hand, lags behind in this important issue.The actual atomic structure of the electrode (and probably that of the CNT) at the contact are unknown and, most likely, change from sample to sample when fabricated under the same conditions. Atomic-scale modeling, however, can still be of guidance to the interpretation of the experiments and to the future design of operational devices with CNT's. In this work we focus on the two key ingredients in this puzzle: The effect the atomic-scale geometry and the chemical nature of the electrode have on the transparency of the contact. We have studied open single-walled metallic (5,5) CNT's contacted in two representative forms (see Fig. 1) to Al, Au, and Ti electrodes which are among the most commonly used metals in the experiments . From our ab-initio transport study we find that in CNT's contacted to Al and Au electrodes for end-contact geometry [see Fig. 1(a)] the two CNT bands couple weakly to the electrodes. This allows us to resolve quasi-bound CNT states in the conductance and to estimate the magnitude of the degeneracy removal due to Coulomb blockade effects in a direct manner. Moreover, we find that the two bands couple very differently to the electrodes (one of them is almost shut down for transport) and do not mix. For the side-contact geometry [see Fig. 1(b)] the coupling is the same for both bands, but similar in strength to the end-contact geometry. Finally, our study presents the first direct numerical evidence of what has been hinted at on the basis of indirect first-principles calculations [11,12] and ...

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 L Ϫ R ϭeV ͑where L and R are the electrochemical potentials well into the left and right electrodes͒, 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.

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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. Chern. Phys. 88, 2547] to decompose a multicenter integral f 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 F(r) dr= 2,jw jF(rj). The set of abscissas rj and weights Wj 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 E, it generates 3D grids which are able to approximate multicenter integrals with relative errors smaller than E 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 E=2.0·1O-3 , E=2.0·1O-4 , E=2.0·1O-5 , E=2.0·1O-6 , and E=2.0·1O-7 is respectively 3141 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.In this paper, we will describe another scheme for the 6520 J.

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρ) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop. We choose to study in detail linear and cyclic oligoacene molecules of increasing size, and relate the emerging differences with the difficulties for the synthesis of the latter due to their radicaloid character. We finally explore how minor structural modifications of the cyclic forms can alter these results, showing the potential use of these systems as molecular templates for the growth of well-shaped carbon nanotubes as well as the usefulness of theoretical tools for molecular design.