Driven Liouville von Neumann Approach for Time-Dependent Electronic Transport Calculations in a Nonorthogonal Basis-Set Representation

A nonorthogonal localized basis-set implementation of the driven Liouville von Neumann (DLvN) approach is presented. The method is based on block-orthogonalization of the Hamiltonian and overlap matrix representations, yielding nonoverlapping blocks that correspond to the various system sections. An extended Hückel description of gold/benzene-dithiol/gold and gold/pyridine-dithiol/gold junctions is used to demonstrate the performance of the method. The presented generalization is an important milestone toward using the DLvN approach for performing accurate dynamic electronic transport calculations in realistic model systems, based on density functional theory packages that rely on atom-centered basis-set representations

Pair-Wise and Many-Body Dispersive Interactions Coupled to an Optimally Tuned Range-Separated Hybrid Functional

We propose a nonempirical, pair-wise or many-body dispersion-corrected, optimally tuned range-separated hybrid functional. This functional retains the advantages of the optimal-tuning approach in the prediction of the electronic structure. At the same time, it gains accuracy in the prediction of binding energies for dispersively bound systems, as demonstrated on the S22 and S66 benchmark sets of weakly bound dimers

Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory

Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theor

Low-Energy Charge-Transfer Excitons in Organic Solids from First-Principles: The Case of Pentacene

The nature of low energy optical excitations, or excitons, in organic solids is of central relevance to many optoelectronic applications, including solar energy conversion. Excitons in solid pentacene, a prototypical organic semiconductor, have been the subject of many experimental and theoretical studies, with differing conclusions as to the degree of their charge-transfer character. Using first-principles calculations based on density functional theory and many-body perturbation theory, we compute the average electron–hole distance and quantify the degree of charge-transfer character within optical excitations in solid-state pentacene. We show that several low-energy singlet excitations are characterized by a weak overlap between electron and hole and an average electron–hole distance greater than 6 Å. Additionally, we show that the character of the lowest-lying singlet and triplet excitons is well-described with a simple analytic envelope function of the electron–hole distance

Interlayer Potential for Homogeneous Graphene and Hexagonal Boron Nitride Systems: Reparametrization for Many-Body Dispersion Effects

A new parametrization of the anisotropic interlayer potential for hexagonal boron nitride (<i>h</i>-BN ILP) is presented. The force-field is benchmarked against density functional theory calculations of several dimer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, corrected for many-body dispersion effects. The latter, more advanced method for treating dispersion, is known to produce binding energies nearly twice as small as those obtained with pairwise correction schemes, used for an earlier ILP parametrization. The new parametrization yields good agreement with the reference calculations to within ∼1 and ∼0.5 meV/atom for binding and sliding energies, respectively. For completeness, we present a complementary parameter set for homogeneous graphitic systems. Together with our previously suggested ILP parametrization for the heterogeneous graphene/<i>h</i>-BN junction, this provides a powerful tool for consistent simulation of the structural, mechanical, tribological, and heat transport properties of both homogeneous and heterogeneous layered structures based on graphene and <i>h</i>-BN

Interlayer Potential for Graphene/<i>h</i>‑BN Heterostructures

We present a new force-field potential that describes the interlayer interactions in heterojunctions based on graphene and hexagonal boron nitride (<i>h</i>-BN). The potential consists of a long-range attractive term and a short-range anisotropic repulsive term. Its parameters are calibrated against reference binding and sliding energy profiles for a set of finite dimer systems and the periodic graphene/<i>h</i>-BN bilayer, obtained from density functional theory using a screened-exchange hybrid functional augmented by a many-body dispersion treatment of long-range correlation. Transferability of the parametrization is demonstrated by considering the binding energy of bulk graphene/<i>h</i>-BN alternating stacks. Benchmark calculations for the superlattice formed when relaxing the supported periodic heterogeneous bilayer provide good agreement with both experimental results and previous computational studies. For a free-standing bilayer we predict a highly corrugated relaxed structure. This, in turn, is expected to strongly alter the physical properties of the underlying monolayers. Our results demonstrate the potential of the developed force-field to model the structural, mechanical, tribological, and dynamic properties of layered heterostructures based on graphene and <i>h</i>-BN

Curvature and Frontier Orbital Energies in Density Functional Theory

Perdew et al. discovered two different properties of exact Kohn–Sham density functional theory (DFT): (i) The exact total energy versus particle number is a series of linear segments between integer electron points. (ii) Across an integer number of electrons, the exchange-correlation potential “jumps” by a constant, known as the derivative discontinuity (DD). Here we show analytically that in both the original and the generalized Kohn–Sham formulation of DFT the two properties are two sides of the same coin. The absence of a DD dictates deviation from piecewise linearity, but the latter, appearing as curvature, can be used to correct for the former, thereby restoring the physical meaning of orbital energies. A simple correction scheme for any semilocal and hybrid functional, even Hartree–Fock theory, is shown to be effective on a set of small molecules, suggesting a practical correction for the infamous DFT gap problem. We show that optimally tuned range-separated hybrid functionals can inherently minimize <i>both</i> DD and curvature, thus requiring no correction, and that this can be used as a sound theoretical basis for novel tuning strategies

Enhanced Magnetoresistance in Molecular Junctions by Geometrical Optimization of Spin-Selective Orbital Hybridization

Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their ferromagnet–molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to ∼200% near the conductance quantum <i>G</i>₀. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal–molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal–molecule interfaces

Molecule-Adsorbed Topological Insulator and Metal Surfaces: A Comparative First-Principles Study

We compare electronic structure characteristics of three different kinds of benzene-adsorbed (111) surfaces: that of Bi₂Te₃, a prototypical topological insulator, that of Au, a prototypical inert metal, and that of Pt, a prototypical catalytic metal. Using first-principles calculations based on dispersion-corrected density functional theory, we show that benzene is chemisorbed on Pt, but physisorbed on Au and Bi₂Te₃. The adsorption on Bi₂Te₃ is particularly weak, consistent with a minimal perturbation of the electronic structure at the surface of the topological insulator, revealed by a detailed analysis of the interaction of the molecular orbitals with the topological surface states

Why Are Diphenylalanine-Based Peptide Nanostructures so Rigid? Insights from First Principles Calculations

The diphenylalanine peptide self-assembles to form nanotubular structures of remarkable mechanical, piezolelectrical, electrical, and optical properties. The tubes are unexpectedly stiff, with reported Young’s moduli of 19–27 GPa that were extracted using two independent techniques. Yet the physical basis for the remarkable rigidity is not fully understood. Here, we calculate the Young’s modulus for bulk diphenylalanine peptide from first principles, using density functional theory with dispersive corrections. The calculation demonstrates that at least half of the stiffness of the material is the result of dispersive interactions. We further quantify the nature of various inter- and intramolecular interactions. We reveal that despite the porous nature of the lattice, there is an array of rigid nanotube backbones with interpenetrating “zipper-like” aromatic interlocks that result in stiffness and robustness. This presents a general strategy for the analysis of bioinspired functional materials and may pave the way for rational design of bionanomaterials