Ángel J. Pérez-Jiménez

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.

Fullerene-based molecular nanobridges: A first-principles study

Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, Madrid 28049, Spain~Received 25 May 2001; published 24 August 2001!Building upon traditional quantum-chemistry calculations, we have implemented an ab initio method tostudy the electrical transport in nanocontacts. We illustrate our technique calculating the conductance of C

Assessment of double-hybrid energy functionals for π-conjugated systems

There have been tremendous efforts in the past decade on the use of computational methods for conjugated systems. Their properties and energetics are often described by density functional theory calculations which, however, are known to face a challenge when dealing with these systems since serious and systematic errors with popular density functionals occur, specially in the case of having stacked or sterically overcrowded aromatic systems, and discourage their use as a black box technique. We overcome here this shortcoming by applying recently developed dispersion-corrected double-hybrid density functionals (B2PLYP) in search of greater yet wide accuracy with little more computational effort. Interestingly, we have derived a related method (B2piPLYP), which has been thoroughly assessed against a set of databases and reactions of the most interest, and works better for this subclass of systems. The deviations with respect to benchmark or experimental values are found to be in the reasonably low range of 1-2 kcal/mol when a correction for the dispersion interactions is added and, most importantly, without suffering the large and systematic errors that are common in former yet conventional methods.

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)].

Communication: Double-hybrid functionals from adiabatic-connection: The QIDH model

A new approach stemming from the adiabatic-connection (AC) formalism is proposed to derive parameter-free double-hybrid (DH) exchange-correlation functionals. It is based on a quadratic form that models the integrand of the coupling parameter, whose components are chosen to satisfy several well-known limiting conditions. Its integration leads to DHs containing a single parameter controlling the amount of exact exchange, which is determined by requiring it to depend on the weight of the MP2 correlation contribution. Two new parameter-free DHs functionals are derived in this way, by incorporating the non-empirical PBE and TPSS functionals in the underlying expression. Their extensive testing using the GMTKN30 benchmark indicates that they are in competition with state-of-the-art DHs, yet providing much better self-interaction errors and opening a new avenue towards the design of accurate double-hybrid exchange-correlation functionals departing from the AC integrand.

Accurate calculation of transport properties for organic molecular semiconductors with spin-component scaled MP2 and modern density functional theory methods

At ambient temperatures, intermolecular hopping of charge carriers dominates the field effect mobility and thus the performance of organic molecular semiconductors for organic-based electronic devices. We have used a wide variety of modern and accurate computational methods to calculate the main parameters associated with charge transport, taking oligoacenes, and its derivatives as the exemplary organic materials. We tackle the problem from a combined inter- and intramolecular approach, in which the parameters are calculated for an isolated single molecule concomitantly with the stability of the dimers found in experimentally determined crystalline structures. Considering that most of the future applications within the field would need a full understanding of the transport mechanism, we assess the reliability of the methods to be employed according to the nature of the problem. Finally, we perform a computationally guided molecular engineering of a new set of materials derived from tetracene (rubrene and highly twisted oligoacenes) which allows to robustly anticipate the reasons for their expected performance in organic-based electronic devices.

Improved accuracy with medium cost computational methods for the evaluation of bond length alternation of increasingly long oligoacetylenes

The difference between the length of the central carbon-carbon bond and that of the adjacent flanked double bonds in polymers such as polyacetylene is closely related to their electronic properties and plays a central role in their conductivity upon doping. Simple as it seems, this bond length alternation (BLA) is a difficult test for many theoretical methods. Accurate coupled-cluster (CC) benchmark values are difficult to obtain even for small- and medium-sized oligoacetylenes due to their intrinsic computational limitations. Here we present a computationally much cheaper alternative to obtain accurate benchmark BLA values, even for large polyacetylene oligomers, by using the so-called spin-component scaled Møller-Plesset perturbation theory up to second order (SCS-MP2). Comparison between these new benchmark BLA with those provided by density functional theory (DFT) calculations shows a large dispersion of the results depending on the amount of exact exchange used in the exchange-correlation functional. We find that the percentage of exact exchange needed to accurately reproduce the new benchmark BLA is much larger than previously thought when comparison was made with values obtained using the MP2 method.

A comparison between DFT and other ab initio schemes on the activation energy in the automerization of cyclobutadiene

Abstract The influence of the multireference character in the transition state for the automerization reaction of cyclobutadiene is considered. We have analyzed two forms of taking into account this effect: either by the use of two-body density functionals or traditional density functional theory (DFT) correlation functionals conveniently modified. Comparison has also been made with conventional density functional theory Kohn–Sham (DFT–KS) [15] approaches. It is shown that only when the aforementioned multideterminantal character is included in the computational scheme is the activation energy in accord with accurate benchmark calculations.

Density functional theory with alternative spin densities: application to magnetic systems with localized spins.

A new method to improve the excess spin density obtained from unrestricted Hartree-Fock wave functions in terms of natural orbitals is proposed. Using this modified excess spin density to evaluate the correlation energy by means of density functionals leads to large improvements in the computed magnetic coupling constants of several materials without need to modify the exchange contribution. This is important because it reconciles the density functional theory description with the one provided by multi-determinant wave functions. Using the present approach, the leading contribution to the magnetic coupling constant arises from electron correlation effects. The performance of the new method is illustrated on various materials including high-critical-temperature superconductors parent compounds.

Ballistic resistivity in aluminum nanocontacts

We thank J. J. Saenz for helpful discussions, and Cecalcula (Venezuela) for computer facilities. This work has been partially supported by the CSIC-IVIC researchers exchange program and the Spanish DGICyT (MEC) through Project No. BFM2003-01167/FISI.