Emilio San-Fabián

First-Principles Phase-Coherent Transport in Metallic Nanotubes with Realistic Contacts

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 CNTs 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 CNTs. Al and Au turn out to make poor contacts while Ti is the best option of the three.

First-principles approach to electrical transport in atomic-scale nanostructures

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 twofacts. 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.

Automatic numerical integration techniques for polyatomic molecules

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≊∑iωiF(ri). The set of abscissas ri and weights ωi 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), an...

DENSITY-FUNCTIONAL STUDY OF VAN DER WAALS FORCES ON RARE-GAS DIATOMICS : HARTREE-FOCK EXCHANGE

A density-functional theory study of van der Waals forces on rare-gas diatomics is carried out. Hartree-Fock-Kohn-Sham formalism is used, that is, the exchange-correlation functional is expressed as the combination of Hartree-Fock exchange plus an approximation to the correlation energy functional. Spectroscopic constants (Re,ωe, and De) and potential energy curves for the molecules He2, Ne2, Ar2, HeNe, HeAr, and NeAr are presented. Several approximations to the correlation functional are tested. The best results, in good agreement with reference experimental data, are obtained with the functional proposed by Wilson and Levy [L. C. Wilson and M. Levy, Phys. Rev. B 41, 12930 (1990)].

A simple, efficient and more reliable scheme for automatic numerical integration

Abstract We have recently proposed a simple, reliable and efficient scheme for automatic numerical integration that uses the change of variable x(z) = 1+(2/Π){[1+ 2 3 (1−z 2 )]z√1−z 2 −arccos z} to transform the integral to be computed, ∫1-1f(x) dx, into (16/3Π) ∫1-1f(x(z)) (1−z2)√1−z2 dz,, which is approximated by successive n-point Gauss-Chebyshev quadrature formulas of the second kind (In). The following sequence of formulas was generated: I1, I3, I7,..., I(n−1)/2, In, I2n+1. In the present work we generate the same sequence, together with another one, I2, I5,...,Im, I 2m+1. Both sequences are generated in an alternate way, I2, I1, I5, 3,...,I(n−1)/2, Im, In, I2m+1, where m + 1 = 2 3 (n + 1) . The main advantage of the new scheme is that, unlike the previous one, the total number of points increases more moderately than doubling. This is possible because all the abscissas of I(n−1)/2 are also abscissas of In, all the abscissas of Im are also abscissas of I2m+1, and, especially, all the abscissas of In are also abscissas of I2m+1. These allows us to implement a simple and efficient algorithm to generate both sequences alternatively (a FORTRAN 77 version is included). We propose an error estimate for the approximate integral that is more conservative than the one used in our previous work, which results in a more reliable automatic numerical integrator, very suitable for multiple integrals.

A simple, reliable and efficient scheme for automatic numerical integration

A scheme for automatic numerical integration is presented. It uses the change of variable x=1+(2π){[1+23(1-z2)]z√1-z2 -arccosz} to transform the integral to be computed, ∫1-1f(x) dx, into (163π)∫1-1f(x)(1-z2)√1-z2 dz, which is approximated by successive n-points Gauss-Chebyshev quadrature formulas of the second kind (In). Due to the special nature of their abscissas and weights, a sequence of formulas I1, I3, I7,…, I(n-1)2, In, I2n+1 may be generated, such that I2n+1 may be computed with only n + 1 new integrand evaluations, using the previous value of In. An error estimation is proposed for I2n+1, which only needs two previous values of the sequence (In and I(n+1)2). The algorithm may be implemented by a very short program (a FORTRAN 77 version is included) that spends practically all its running time in integrand evaluations. It is compared with other methods for automatic numerical integration (trapezoidal rule, Simpsons rule, Rombergs method, an adaptive Gauss-Kronrod rule and Clenshaw-Curtis method) over a broad set of 20 functions. We conclude that the present method is very simple and reliable and is the most efficient among the methods tested here. Possible applications in density functional theory are explored.

Electronic transport and vibrational modes in a small molecular bridge: H 2 in Pt nanocontacts

We present a state-of-the-art first-principles analysis of electronic transport in a Pt nanocontact in the presence of

New approach to the design of density functionals

{\mathrm{H}}_{2}

Intra‐ and Intermolecular Dispersion Interactions in [n]Cycloparaphenylenes: Do They Influence Their Structural and Electronic Properties?

which has been recently reported by Smit et al. [Nature (London) 419, 906 (2002)]. Our results indicate that at the last stages of the breaking of the Pt nanocontact two basic forms of bridge involving H can appear. Our claim is, in contrast to the claim of Smit et al., that the main conductance histogram peak at

Electron correlation in the Coulomb hole model. Comparison of methods

G\ensuremath{\approx}{2e}^{2}/h