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.

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.

Building upon traditional quantum chemistry calculations, we have implemented an ab-initio method to study the electrical transport in nanocontacts. We illustrate our technique calculating the conductance of C60 molecules connected in various ways to Al electrodes characterized at the atomic level. Central to a correct estimate of the electrical current is a precise knowledge of the local charge transfer between molecule and metal which, in turn, guarantees the correct positioning of the Fermi level with respect to the molecular orbitals. Contrary to our expectations, ballistic transport seems to occur in this system.

In this letter we report the error analysis of 59 exchange-correlation functionals in evaluating the structural parameters of small-and medium-sized organic molecules.From this analysis, recently developed double-hybrids, such as xDH-PBE0, emerge as the most reliable methods, while global-hybrids confirm their robustness in reproducing molecular structures. Notably the M06-L density-functional is the only semilocal method reaching an accuracy comparable to hybrids'. A comparison with errors obtained on energetic databases (including thermochemistry, reaction barriers and interaction energies) indicate that most of the functionals have a coherent behavior, showing low (or high) deviations on both energy and structure datasets. Only a few of them are more prone toward one of these two properties. 2The quality of any method rooted in density functional theory (DFT) is (strongly) affected by the choice of the exchange-correlation functional (ECF), which gives the unknown term of the Kohn-Sham energy. If from one side the spreading of DFT in chemistry and physics has encouraged the research of new and better-performing density-functionals, 1 from the other side their validation has become a due step before any routine application. Such a benchmark passes through a careful evaluation (and consequent statistical analysis) of the errors on defined properties and systems sets.Starting from the nineties, a large effort has been made in order to define standard benchmark sets allowing for a meaningful and fair comparison between different ECFs. 2-8Among the properties firstly targeted, atomization energies, ionization potentials and electron affinities 2-4 as well as bond lengths and angles of (mostly) small organic systems received a particular attention. Figure S1 and S2 in the Supporting Information). Both databases are an excellent diagnostic test to discriminate density-functionals in modeling structural parameters of organic systems.In this Letter, we use these two datasets to thoroughly benchmark the accuracy of 59ECFs (reported in Table The references and further details of all the considered computational methods involved in this Letter are given in Table S1 of the Supporting Information.In order to discriminate the accuracy of the selected approaches, we define a criterion based on the matrix containing all the interatomic distances. For each system, we compute the mean absolute deviation (MAD) over the distance matrix of the probed and the reference geometries, and calculate the averaged deviation over the set. Figure 1 reports these statistics for the 63 computational approaches considered in this Letter (see Table S2 and S3 in the Supporting Information for more details).For the CCse21 dataset, the deviations span from 0.002 to 0.016 Å for xDH-PBE0and HF methods, respectively. Within this interval, a smooth transition from high to low accuracy is observed. Apart from the worst performing ECFs like BLYP, B97D, B97D3 or TPSS, most of the methods give a slight increase of the distance matrix deviation (...

On the basis of our previous developments in the field of nonempirical double hybrids, we present here a new exchange-correlation functional based on a range-separated model for the exchange part and integrating a nonlocal perturbative correction to the electron correlation contribution. Named RSX-QIDH, the functional is free from any kind of empirical parametrization. Its range-separation parameter is set to recover the total energy of the hydrogen atom, thus eliminating the self-interaction error for this one-electron system. Subsequent tests on some relevant benchmark data sets confirm that the self-interaction error is particularly low for RSX-QIDH. This new functional provides also correct dissociation profiles for charged rare-gas dimers and very accurate ionization potentials directly from Kohn-Sham orbital energies. Above all, these good results are not obtained at the expense of other properties. Indeed, further tests on standard benchmarks show that RSX-QIDH is competitive with the more empirical ωB97X-2 double hybrid and outperforms the parent LC-PBE long-range corrected hybrid, thus underlining the important role of the nonlocal perturbative correlation.

In this communication, we present a new and simple route to derive range-separated exchange (RSX) hybrid and double hybrid density functionals in a nonempirical fashion. In line with our previous developments [Brémond et al., J. Chem. Theory Comput. 14, 4052 (2018)], we show that by imposing an additional physical constraint to the exchange-correlation energy, i.e., by enforcing to reproduce the total energy of the hydrogen atom, we are able to generalize the nonempirical determination of the range-separation parameter to a family of RSX hybrid density functionals. The success of the resulting models is illustrated by an accurate modeling of several molecular systems and properties, like ionization potentials, particularly prone to the one-and many-electron self-interaction errors.

The torsional potential of 1,3-butadiene has been calculated using several ab initio methodologies. For each value of the CdC-CdC torsional angle, the fully relaxed geometry and energy have been determined using the Hartree-Fock (HF) method, Mo ¨eller-Plesset perturbation theory up to the second order (MP2), and the coupled-cluster method with single, doubles, and parethentical triples (CCSD(T)), as well as using several exchange-correlation combinations of functionals in density functional theory (DFT) calculations. From the results obtained, the achievements and drawbacks of current density functionals in the description of torsional profiles have been rationalized, and some possible breakthroughs have been proposed to improve their performance.

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 ...