Search

A formally exact density functional theory (DFT) determination of the average electron energy is presented. Our theory, which is based on a different accounting of energy functional terms, partially solves one well known downside of conventional Kohn-Sham (KS) DFT: that electronic energies have but tenuous connections to physical quantities. Calculated average electron energies are close to experimental ionization potentials in one-electron systems, demonstrating a surprisingly small effect of self-interaction and other exchange-correlation errors in established DFT methods. Remarkable agreement with ab initio quantum mechanical calculations of multi-electron systems is demonstrated using several flavors of DFT, and we argue for the use of the average electron energy as a design criterion for density functional approximations.

Diamine-functionalized M

The all-inorganic perovskite barium zirconate, BaZrO3, is a widely used material in a range of different technological applications. However, fundamental questions surrounding the crystal structure of BaZrO3, especially in regard to its ground-state structure, remain. While diffraction techniques indicate a cubic structure all the way down to T = 0 K, several first-principles phonon calculation studies based on density functional theory indicate an imaginary (unstable) phonon mode due to the appearance of an antiferrodistortive transition associated with rigid rotations of ZrO6 octahedra. The first-principles calculations are highly sensitive to the choice of exchange-correlation functional and, using six well-established functional approximations, we show that a correct description about the ground-state structure of BaZrO3 requires the use of hybrid functionals. The ground-state structure of BaZrO3 is found to be cubic, which is corroborated by experimental results obtained from neutron powder diffraction, inelastic neutron scattering, and neutron Compton scattering experiments.

We present a formally exact density functional theory (DFT) determination of the average electron energy. Our orbital-free theory, which is based on a different accounting of energy functional terms, partially solves one well known downside of Kohn-Sham (KS) DFT: that orbital energies have but tenuous connections to physical quantities. Our computed average electron energies are close to experimental ionization potentials in one-electron systems, demonstrating a surprisingly small effect of self-interaction and other exchange- correlation errors in established DFT methods. We argue for the use of the average electron energy as a physics condition on otherwise fictitious KS energy levels and as a design criterion for density functional approximations.

We explore the possibility and potential benefit of rolling a Si2BN sheet into single-walled nanotubes (NTs). Using density functional theory (DFT), we consider both structural stability and the impact on the nature of chemical bonding and conduction. The structure is similar to carbon NTs and hexagonal boron-nitride (hBN) NTs and we consider both armchair and zigzag Si2BN configurations with varying diameters. The stability of these Si$_2$BN NTs is confirmed by first-principles molecular dynamics calculations, by an exothermal formation, an absence of imaginary modes in the phonon spectra. Also, we find the nature of conduction varies semiconducting, from semi-metallic to metallic, reflecting differences in armchair/zigzag-type structures, curvature effects, and the effect of quantum confinement. We present the detailed characterization of how these properties lead to differences in both the bonding nature and electronic structures, 14 pages, 8 figures

We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van der Waals density functional (vdW-DF) method [JPCM 32, 393001 (2020)]. Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: magnetic elements, perovskites, and biomolecular problems. We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we furthermore present a stress formulation for spin vdW-DF and illustrate use of a simple stability-modeling scheme to assert when the prediction of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies a prediction of an actual low-temperature transformation., 23 Pages, 8 figures

The ground-stale structure of BaZrO3 is experimentally known to be cubic down to absolute zero. However, there exist several measured properties and experimental characterizations that earlier computational works have failed to accurately describe and explain within this cubic symmetry. Among these properties and observations are the dielectric constant and the parallel mean-squared relative displacement value that tracks the fluctuations in distance for Ba-O atom pairs. Previous density-functional theory (DFT) studies have resolved the issue by assuming that BaZrO(3 )undergoes a phase transition from cubic to tetragonal I4/mcm symmetry, possibly while forming a glasslike state that reflects cubic symmetry on average. In this paper, we show that the set of experimental results can indeed be satisfactorily explained by DFT entirely within the cubic symmetry. We find that past theory limitations arose from the choice of exchange-correlation-functional approximations and that the inclusion of Fock exchange in hybrids significantly improves the DFT performance. We also find that the inclusion of nonlocal correlation effects is beneficial. We conclude by making a prediction for the phase-transition pressure for the transition from cubic to tetragonal symmetry at zero kelvin.

We review the screening nature and many-body physics foundation of the van der Waals density functional (vdW-DF) method, a systematic approach to construct truly nonlocal exchange-correlation energy density functionals. To that end we define and focus on a class of consistent vdW-DF versions that adhere to the Lindhard screening logic of the full method formulation. The consistent-exchange vdW-DF-cx version and its spin extension represent the first examples of this class; In general, consistent vdW-DFs reflect a concerted expansion of a formal recast of the adiabatic-connection formula, an exponential summation of contributions to the local-field response, and the Dyson equation. We argue that the screening emphasis is essential because the exchange-correlation energy reflects an effective electrodynamics set by a long-range interaction. Two consequences are that 1) there are, in principle, no wiggle room in how one balances exchange and correlation, for example, in vdW-DF-cx, and that 2) consistent vdW-DFs have a formal structure that allows them to incorporate vertex-correction effects, at least in the case of levels that experience recoil-less interactions (for example, near the Fermi surface). We explore the extent to which the strictly nonempirical vdW-DF-cx formulation can serve as a systematic extension of the constraint-based semilocal functionals. For validation, we provide a complete survey of vdW-DF-cx performance for broad molecular processes and comparing to the quantum-chemistry calculations that are summarized in that paper. We also provide new vdW-DF-cx results for metal surface energies and work functions that we compare to experiment. Finally, we use the screening insight to separate the vdW-DF nonlocal-correlation term and present tools to compute and map the binding signatures., 57 pages, 16 figures, 4 Tables. Published as an invited Topical review in IOP Journal of Physics: Condensed Matter

In this work, we systematically investigate and compare noninvasive doping of chemical vapor deposition graphene with three molecule dopants through spectroscopy and electrical conductivity techniques. Thionyl chloride shows the smallest improvement in conductivity with poor temporal and thermal stability and nitric acid induces the biggest sheet resistance reduction with modified stability. Molybdenum trioxide doping stands out, after thermal annealing, with both causing a significant sheet-resistance reduction and having superior temporal and thermal stability. These properties make it ideal for applications in advanced electronics. Theoretical studies based on the van der Waals density functional method suggest that cluster formation of molybdenum trioxide underpins the significant reduction in sheet resistance, and the stability, that arises after thermal annealing. Our comparative study clarifies charge transfer doping of graphene and brings understanding of the weak-interaction nature of such non-destructive doping of graphene. Our work also shows that we can use weak chemisorption to tailor the electronic properties of graphene, for example, to improve conductivity. This ability open up possibilities for further use of graphene in electronic interconnects, field effect transistors and other systems.

Molecular photoswitches capable of storing solar energy are interesting candidates for future renewable energy applications. Here, using quantum mechanical calculations, we carry out a systematic screening of crucial optical (solar spectrum match) and thermal (storage energy density) properties of 64 such compounds based on the norbornadiene-quadricyclane system. Whereas a substantial number of these molecules reach the theoretical maximum solar power conversion efficiency, this requires a strong red-shift of the absorption spectrum, which causes undesirable absorption by the photoisomer as well as reduced thermal stability. These compounds typically also have a large molecular mass, leading to low storage densities. By contrast, single-substituted systems achieve a good compromise between efficiency and storage density, while avoiding competing absorption by the photo-isomer. This establishes guiding principles for the future development of molecular solar thermal storage systems.

Molecular photoswitches that are capable of storing solar energy, so-called molecular solar thermal storage systems, are interesting candidates for future renewable energy applications. In this context, substituted norbornadiene-quadricyclane systems have received renewed interest due to recent advances in their synthesis. The optical, thermodynamic, and kinetic properties of these systems can vary dramatically depending on the chosen substituents. The molecular design of optimal compounds therefore requires a detailed understanding of the effect of individual substituents as well as their interplay. Here, we model absorption spectra, potential energy storage, and thermal barriers for back-conversion of several substituted systems using both single-reference (density functional theory using PBE, B3LYP, CAM-B3LYP, M06, M06-2x, and M06-L functionals as well as MP2 calculations) and multireference methods (complete active space techniques). Already the diaryl substituted compound displays a strong red-shift compared to the unsubstituted system, which is shown to result from the extension of the conjugated π-system upon substitution. Using specific donor/acceptor groups gives rise to a further albeit relatively smaller red-shift. The calculated storage energy is found to be rather insensitive to the specific substituents, although solvent effects are likely to be important and require further study. The barrier for thermal back-conversion exhibits strong multireference character and as a result is noticeably correlated with the red-shift. Two possible reaction paths for the thermal back-conversion of diaryl substituted quadricyclane are identified and it is shown that among the compounds considered the path via the acceptor side is systematically favored. Finally, the present study establishes the basis for high-throughput screening of norbornadiene-quadricyclane compounds as it provides guidelines for the level of accuracy that can be expected for key properties from several different techniques.

The vdW-DF-cx0 exchange-correlation hybrid design has a truly nonlocal correlation component and aims to facilitate concurrent descriptions of both covalent and non-covalent molecular interactions. The vdW-DF-cx0 design mixes a fixed ratio, $a$, of Fock exchange into the consistent-exchange van der Waals density functional, vdW-DF-cx. The mixing value $a$ is sometimes taken as a semi-empirical parameter in hybrid formulations. Here, instead, we assert a plausible optimum average $a$ value for the vdW-DF-cx0 design from a formal analysis; A new, independent determination of the mixing $a$ is necessary since the Becke fit, yielding $a'=0.2$, is restricted to semilocal correlation and does not reflect non-covalent interactions. To proceed, we adapt the so-called two-legged hybrid construction to a starting point in the vdW-DF-cx functional. For our approach, termed vdW-DF-tlh, we estimate the properties of the adiabatic-connection specification of the exact exchange-correlation functional, by combining calculations of the Fock exchange and of the coupling-constant variation in vdW-DF-cx. We find that such vdW-DF-tlh hybrid constructions yield accurate characterizations of molecular. The accuracy motivates trust in the vdW-DF-tlh determination of system-specific values of the Fock-exchange mixing. We find that an average value $a'=0.2$ best characterizes the vdW-DF-tlh description of covalent and non-covalent interactions, although there exists some scatter. This finding suggests that the original Becke value, $a'=0.2$, also represents an optimal average Fock-exchange mixing for the new, truly nonlocal-correlation hybrids. To enable self-consistent calculations, we furthermore define and test a zero-parameter hybrid functional vdW-DF-cx0p (having fixed mixing $a'=0.2$) and document that this truly nonlocal correlation hybrid works for general molecular interactions., 18 pages, 5 figures, accepted by J. Chem. Phys

The equilibrium van der Waals binding energy is an important factor in the design of materials and devices. However, it presents great computational challenges for materials built up from nanostructures. Here we investigate the binding-energy scaling behavior from first-principles calculations. We show that the equilibrium binding energy per atom between identical nanostructures can scale up or down with nanostructure size, but can be parametrized for large N with an analytical formula (in meV/atom), Eb/N=a+b/N+c/N2+d/N3, where N is the number of atoms in a nanostructure and a, b, c, and d are fitting parameters, depending on the properties of a nanostructure. The formula is consistent with a finite large-size limit of binding energy per atom. We find that there are two competing factors in the determination of the binding energy: Nonadditivities of van der Waals coefficients and center-to-center distance between nanostructures. To decode the detail, the nonadditivity of the static multipole polarizability is investigated from an accurate spherical-shell model. We find that the higher-order multipole polarizability displays ultrastrong intrinsic nonadditivity, no matter if the dipole polarizability is additive or not.

We study the phase stability and martensitic transformation of orthorhombic and monoclinic polyethylene by means of density functional theory using the non-empirical consistent-exchange vdW-DF-cx functional [Phys. Rev. B $\mathbf{89}$, 035412 (2014)]. The results show that the orthorhombic phase is the most stable of the two. Owing to the occurrence of soft librational phonon modes, the monoclinic phase is predicted not to be stable at zero pressure and temperature, but becomes stable when subjected to compressive transverse deformations that pin the chains and prevent them from wiggling freely. This theoretical characterization, or prediction, is consistent with the fact that the monoclinic phase is only observed experimentally when the material is subjected to mechanical loading. Also, the estimated threshold energy for the combination of lattice deformation associated with the T1 and T2 transformation paths (between the orthorhombic and monoclinic phases) and chain shuffling is found to be sufficiently low for thermally activated back transformations to occur. Thus, our prediction is that the crystalline part can transform back from the monoclinic to the orthorhombic phase upon unloading and/or annealing, which is consistent with experimental observations. Finally, we observe how a combination of such phase transformations can lead to a fold-plane reorientation from {110} to {100}-type in a single orthorhombic crystal.

Films of layered substances like WSe2 can exhibit a reduction in the out-of-plane thermal conductivity of more than 1 order of magnitude compared to that of the bulk, effectively beating the glass limit (Science 2007, 315, 351). Here, we investigate the microscopic contributions that govern this behavior within the framework of Boltzmann transport theory informed by first-principles calculations. To quantitatively reproduce both the magnitude and the temperature dependence of the experimental data, one must account for both phonon confinement effects (softening and localization) and interlayer scattering. Both stacking order and layer spacing are shown to have a pronounced effect on the thermal conductivity that could be exploited to tune the balance between electrical and thermal conductivity.

The van der Waals (vdW) density functional (vdW-DF) method [ROPP 78, 066501 (2015)] describes dispersion or vdW binding by tracking the effects of an electrodynamic coupling among pairs of electrons and their associated exchange-correlation holes. This is done in a nonlocal-correlation energy term $E_c^{nl}$, which permits density functional theory calculation in the Kohn-Sham scheme. However, to map the nature of vdW forces in the fully interacting materials system, it is necessary to compensate for associated kinetic-correlation energy effects. Here we present a coupling-constant scaling analysis that also permits us to compute the kinetic-correlation energy $T_c^{nl}$ that is specific to the vdW-DF account of nonlocal correlations. We thus provide a spatially-resolved analysis of the total nonlocal-correlation binding, including vdW forces, in both covalently and non-covalently bonded systems. We find that kinetic-correlation energy effects play a significant role in the account of vdW or dispersion interactions among molecules. We also find that the signatures that we reveal in our full-interaction mapping are typically given by the spatial variation in the $E_c^{nl}$ binding contributions, at least in a qualitative discussion. Furthermore, our full mapping shows that the total nonlocal-correlation binding is concentrated to pockets in the sparse electron distribution located between the material fragments., 15 pages, 8 figures

We assess the performance of nonempirical, truly nonlocal and semi-local functionals with regard to structural and thermal properties of $3d$, $4d$, and $5d$ non-magnetic transition metals. We focus on constraint-based functionals and consider the new consistent-exchange van der Waals density functional version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys. Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100, 136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108 (2005)]. Using the quasi-harmonic approximation structural parameters, elastic response, and thermal expansion at finite temperatures are computed and compared to experimental data. We also compute cohesive energies explicitly including zero-point vibrations. It is shown that overall vdW-DF-cx provides an accurate description of thermal properties and retains a level of transferability and accuracy that is comparable to or better than some of the best constraint-based semi-local functionals. Especially, with regard to the cohesive energies the consistent inclusion of spin polarization effects in the atoms turns out to be crucial and it is important to use the rigorous spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This demonstrates that vdW-DF-cx has general-purpose character and can be used to study systems that have both sparse and dense electron distributions., 10 pages, 5 figures

van der Waals interactions are a phenomenon where charge fluctuations in one part of a system correlate with fluctuations in another, resulting in an attractive force. Such interactions are thus a truly non-local correlation effect. While the full—albeit unknown—density functional does include these interactions, standard local and semi-local density functionals cannot account for these non-local effects by construction and yield qualitatively erroneous predictions. The simplest expression of a non-local functional of the density rho(r) takes the form Integral[d3r d3r' rho(r) phi(r,r') rho(r')], but it was not until the end of the last century that the means to find a physically motivated, general, and transferable kernel rho emerged. The present chapter discusses the work on this kernel rho leading to the development of the successful van der Waals density functional (vdW-DF) and its variants.

We study the stability of twin boundaries and slip in crystalline orthorhombic polyethylene by means of density functional theory (DFT), using a nonempirical, truly nonlocal density function, and by means of classical molecular dynamics (MD). The results show that, in accordance with experimental observations, there is a clear preference to chain slip over transverse slip for all considered slip planes. The activation energy for pure chain slip lies in the range 10–20 mJ/m2 while that for transverse slip corresponds to 40–280 mJ/m2. For the (11¯0)-slip plane the energy landscape is non-convex with multiple potential energy minima, indicating the presence of stable stacking faults. This suggests that dissociation of perfect dislocations into partials may occur. For the two low-energy twin boundaries considered in this work, {110} and {310}, we find that the former is more stable than the latter, with ground state energies corresponding to 8.9 and 28 mJ/m2, respectively. We have also evaluated how well the empirical MD simulations with the all-atom optimized potential for liquid MD simulations (OPLS-AA) and the coarse-grained united atom (UA) potential concur with the DFT results. It is found that an all-atom potential is necessary to partially capture the γ-surface energy landscapes obtained from the DFT calculations. The OPLS-AA predicts chain slip activation energies comparable with DFT data, while the transverse slip energy thresholds are low in comparison, which is attributed to weak close ranged monomer repulsion. Finally, we find that the H-H interaction dominates the slip activation. While not explicitly represented in the UA potential, its key role is revealed by correlating the DFT energy landscape with changes in the electron distributions and by MD simulations in which components of the OPLS-AA intermolecular potential are selectively silenced. © 2017 Elsevier Ltd

Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with (i) the consistent-exchange vdW-DF-cx functional [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] and (ii) with the vdW-DF2 functional [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. The ability to describe covalent and non-covalent binding properties of molecules is assessed. For properties related to covalent binding, atomization energies (G2-1 set), molecular reaction energies (G2RC set), and ionization energies (G21IP set) are benchmarked against experimental reference values. We find that hybrid-vdW-DF-cx yields results that are rather similar to those of the standard non-empirical hybrid PBE0 [C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999)], with mean average deviations (MADs) of 4.9 and 5.0 kcal/mol for the G2-1 set, respectively. In this comparison, experimental reference values are used, back corrected by wavefunction-based quantum-chemistry calculations of zero-point energies. Hybrid vdW-DF2 follows somewhat different trends, showing on average significantly larger deviations from the reference energies, with a MAD of 14.5 kcal/mol for the G2-1 set. Non-covalent binding properties of molecules are assessed using the S22 benchmark set of non-covalently bonded dimers and the X40 set of dimers of small halogenated molecules, using wavefunction-based quantum chemistry results as references. For the S22 set, hybrid-vdW-DF-cx performs better than standard vdW-DF-cx for the mostly hydrogen-bonded systems, with MAD dropping from 0.6 to 0.3 kcal/mol, but worse for purely dispersion-bonded systems, with MAD increasing from 0.2 to 0.6 kcal/mol. Hybrid-vdW-DF2 offers a slight improvement over standard vdW-DF2. Similar trends are found for the X40 set, with hybrid-vdW-DF-cx performing particularly well for binding energies involving the strongly polar hydrogen halides, but poorly for systems with tiny binding energies. Our study of the X40 set reveals the potential of mixing Fock exchange with vdW-DF, but also highlights shortcomings of the hybrids constructed here. The solid performance of hybrid-vdW-DF-cx for covalent-bonded systems, as well as the strengths and issues uncovered for non-covalently bonded systems, makes this study a good starting point for developing even more accurate hybrid vdW-DFs. The final version of this research has been published in the Journal of Chemical Physics. © 2017 AIP Publishing

We present libvdwxc, a general library for evaluating the energy and potential for the family of vdW-DF exchange--correlation functionals. libvdwxc provides an efficient implementation of the vdW-DF method and can be interfaced with various general-purpose DFT codes. Currently, the GPAW and Octopus codes implement interfaces to libvdwxc. The present implementation emphasizes scalability and parallel performance, and thereby enables \textit{ab initio} calculations of nanometer-scale complexes. The numerical accuracy is benchmarked on the S22 test set whereas parallel performance is benchmarked on ligand-protected gold nanoparticles ($\text{Au}_{144}(\text{SC}_{11}\text{NH}_{25})_{60}$) up to 9696 atoms., Comment: 19 pages, 3 figures

Molecular crystals are a prototypical class of van der Waals (vdW) bound organic materials with excited state properties relevant for optoelectronics applications. Predicting the structure and excited state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) do not capture long range vdW dispersion interactions and do not yield excited state properties. In this work, we use a combination of DFT including vdW forces) using both non local correlation functionals and pair wise correction methods (together with many body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire series of oligoacene crystals, from benzene to hexacene. We find that vdW methods can predict lattice constants within 1 percent of the experimental measurements, on par with the previously reported accuracy of pairwise approximations for the same systems. We further find that excitation energies are sensitive to geometry, but if optimized geometries are used MBPT can yield excited state properties within a few tenths of an eV from experiment. We elucidate trends in MBPT computed charged and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.

Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.

This paper investigates the interfacial structure in hot-wall CVD TiN/kappa-Al2O3 multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the kappa-Al2O3 layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/kappa-Al2O3 coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/kappa-Al2O3 coatings, are not present. However, the orientation relationships at the TiN/kappa-Al2O3 interfaces are the same in both multilayers. The HREM investigations show that kappa-Al2O3 (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, gamma-Al2O3 can be grown, at least partly stabilized by the cube-on-cube orientation relationship between gamma-Al2O3 and the underlying TiN. The DFT calculations show very similar adsorption strengths for an 0 monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fee site. 0 adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for kappa-Al2O3 on TiN(111) than on TiC(111) and a higher stability for kappa-Al2O3 than for alpha-Al2O3 on both TiC and TiN. This indicates that the observed higher stability Of kappa-Al2O3 on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of kappa-Al2O3 over alpha-Al2O3 can be partly attributed to the mismatch.

We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal-correlation driven adsorption of H$_2$ and CO$_2$ in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO$_2$ adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO$_2$ would suggest. We explain this counter-intuitive result, tracking the behavior to a coincidental hybridization of the O $p$ states with the Ni $d$ states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions., Comment: Phys. Rev. Lett., in print (2015)

Carbon nanostructures are promising candidates for ultra-small electronics, but the lack of a cost-effective approach for large scale production also constitutes an important problem. Here, in a pilot study, we describe and illustrate a surface-based assembly that involves three steps: (a) the self-organization of a highly reactive one-dimensional surface electron system, (b) a strong and orientational-specific anchoring of graphite overlayers, followed possibly by (c) (stimulated) exfoliation and rebonding of the graphite sheets into nanotubes. We present preliminary results to characterize this approach. We argue that the nanotube chirality and hence the nature of electronic conduction are prespecified by the surface electron structure. The discussion focuses on the one-dimensional metallic electron gas (1DEG) systems that we have recently predicted to form on flexible and low-symmetry oxides [Phys. Rev. Lett. 90 (2003) 236803]; here the anchoring is due to metallic chemisorption. The proposed nanostructure fabrication method can be developed to provide in situ assembly of circuits of nanostructures between predefined contacts and near gates.

Systematic studies of O adsorption on clean and H-saturated Si-rich 3C–SiC(0 0 1) 3 × 2 surfaces within density functional theory are presented. We investigate the O binding energy for a variety of possible adsorption sites on the surface and in subsurface regions for both substrates. We find that the on-surface adsorption sites are preferred over deep adsorption for both substrates and that O is more strongly bound on the hydrogenated surface. We explore the dependence between the energy of the adsorption site and the surface relaxation accompanying it.

The details of a density functional that includes van der Waals (vdW) interactions are presented. In particular we give some key steps of the transition from a form for fully planar systems to a procedure for realistic layered compounds that have planar symmetry only on large-distance scales, and which have strong covalent bonds within the layers. It is shown that the random-phase approximation of that original functional can be replaced by an approximation that is exact at large separation between vdW interacting fragments and seamless as the fragments merge. An approximation to the latter which renders the functional easily applicable and which preserves useful accuracy in both limits and in between is given. We report additional data from applications to forms of graphite, boron nitride, and molybdenum sulfide not reported in our previous communication.(C) 2004 Wiley Periodicals, Inc.

Low-temperature scanning tunneling microscopy was used to assemble monatomic Cu chains on Cu(1 1 1) by atomic manipulation at 7 K and to explore their electronic structure. Tunneling spectroscopy reveals the formation of unoccupied chain-localized states in the pseudogap of the projected Cu bulk bandstructure. The dispersion of these states is fully described by tight binding behavior. Density functional theory calculations confirm the formation of unoccupied states in the relevant energy regime and in addition predict the existence of occupied states trapped in the pseudogap.

For sparse materials like graphitic systems and carbon nanotubes the standard density functional theory (DFT) faces significant problems because it cannot accurately describe the van der Waals interactions that are essential to the carbon-nanostructure materials behavior. While standard implementations of DFT can describe the strong chemical binding within an isolated, single-walled carbon nanotube, a new and extended DFT implementation is needed to describe the binding between nanotubes. We here provide the first steps to such an extension for parallel and concentric nanotubes through an electron-density based description of the materials coupling to the electrodynamical field. We thus find a consistent description of the (fully screened) van der Waals interactions that bind the nanotubes across the low-electron-density voids between the nanotubes, in bundles and as multiwalled tubes.

Graphitic systems are ideal examples of the soft-matter challenge for quantum mechanics that must account for both strong local atom bonds and weak nonlocal van der Waals interactions between atoms separated by empty space. Here a new density functional (DF), which encompasses nonlocal correlations among the electrons, is applied successfully to obtain structure, bonding, and compressibility, documenting the decisive role of the interlayer vdW interactions. The understanding emerging from this new extended-DF formulation has bearing on broad classes of problems, including surfaces, in physics, chemistry, biology, medicine, and technology, where we can now provide an efficient and accurate DF-theory account.

The interaction energy of a three-adsorbate cluster on a surface consists of the sum of pair interactions plus a trio contribution. The trio interaction is produced by interference of electrons which propagate around the cluster perimeter, d123. We investigate such interactions for a noble-metal (1 1 1) surface, where the isotropic Shockley surfacestate band produces cluster-interaction energies significant for the low-temperature adsorbate dynamics. We provide experimentally testable interaction-energy predictions, specified by the s-wave phase shift dF that characterizes the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate-pair interactions, the trio contribution exhibits a slightly weaker amplitude and a slightly faster asymptotic decay. We discuss howthe trio interaction can affect the cluster geometry by introducing an additional angular dependence in the adsorbate interaction. 2003 Elsevier Science B.V. All rights reserved.

The interaction energy of three adsorbates on a surface consists of the sum of the three-pair interactions plus a trio contribution produced primarily by interference of electrons which traverse the entire perimeter, d123, of the three-adsorbate cluster. Here, we investigate this three-adatom interaction when mediated by the isotropic Shockley surface-state band found on noble-metal (1 1 1) surfaces, extending work on pair interactions. Our experimentally testable result depends on the s-wave phase-shift, characterizing the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate–pair interactions, and in contrast to bulk-mediated interactions, the trio contribution exhibits a slightly weaker amplitude and a slightly faster asymptotic envelope decay, d123−5/2. It also has a different but well-defined oscillation period dependent on d123 and little dependence on the shape of the cluster. We finally compare the asymptotic description with exact model calculations assuming short-range interactions, which are viable even in the non-asymptotic range (when not outweighed by bulk-mediated interactions).

The interaction energy of three adsorbates on a surface consists of the sum of the three pair interactions plus a trio contribution produced by interference of electrons which propagate the entire perimeter, d123, of the three-adsorbate cluster. Here we investigate this triple-adsorbate interaction that is mediated by the isotropic Shockley surface-state band found on noble-metal (111) surfaces. Our experimentally testable result depends on the s-wave phase shift, δF ≠ 0, characterizing the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate-pair interactions, and in contrast to bulk-mediated interactions, the trio contribution has a slightly weaker amplitude and asymptotically decays slightly faster, d123−5/2. It also has a distinctive oscillation period dependent on d123. We finally compare the asymptotic description with exact model calculations.

Waveguide designs for AlGaN-based near-infrared quantum cascade lasers are investigated using a finite-difference method mode solver. Because of the negligibly small refractive index difference between the AlN/GaN/AlGaN gain region and the surrounding AlGaN current injection and extraction layers, a low refractive index substrate (sapphire) and a low refractive index dielectric (SiO2) are used for vertical confinement of the optical field. A ridge waveguide with an off-center contact metallization is used for lateral confinement. The off-center contact allows for the propagation of a TM mode with low metal induced loss and sufficient optical confinement in the gain region. A viable waveguide design with a metal-induced loss of 6.1 cm-1 and a confinement factor of 0.52 is demonstrated.

The nonlocal correlation energy in the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004); Phys. Rev. B 76, 125112 (2007); Phys. Rev. B 89, 035412 (2014)] can be interpreted in terms of a coupling of zero-point energies of characteristic modes of semilocal exchange-correlation (xc) holes. These xc holes reflect the internal functional in the framework of the vdW-DF method [Phys. Rev. B 82, 081101(2010)]. We explore the internal xc hole components, showing that they share properties with those of the generalized-gradient approximation. We use these results to illustrate the nonlocality in the vdW-DF description and analyze the vdW-DF formulation of nonlocal correlation., 13 pages, 6 figures. Submited to Physical Review B

Is the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) robust enough to describe all exchange-correlation components? To address this question, we design an exchange functional based on this plasmon description as well as recent analysis on exchange in the large-s regime. In the regime with reduced gradients s = |del n|/2nk(F)(n) smaller than approximate to 2.5, dominating the nonlocal correlation part of the binding energy, the enhancement factor F-x(s) closely resembles the Langreth-Vosko screened exchange. In the s regime beyond, dominated by exchange, F-x(s) passes smoothly over to the revised Perdew-Wang-86 form. We term the specific exchange functional LV-PW86r, wheras the full van der Waals functional version emphasizing consistent handling of exchange is termed vdW-DF-cx. Our tests indicate that vdW-DF-cx produces accurate separations and binding energies of the S22 data set of molecular dimers as well as accurate lattice constants and bulk moduli of layered materials and tightly bound solids. These results suggest that the plasmon description within vdW-DF gives a good description of both exchange and correlation effects in the low-to-moderate s regime.

A fullerene-tube heterostructure should produce a compact resonant-tunneling system, when a nanometer section of nonconducting nanotube separates a pair of metallic-nanotubes leads. Adding a set of metal contacts and gates completes our proposal for a robust current switch and transistor with a nanosized feature size. The electrostatic gates provide a crisp resonance-level control for the orbitals trapped in the nonconducting barrier and can thus selectively enable/inhibit a strong resonant-tunneling current. A conserving Green-function study documents that the transistor effect is robust at room temperature and in the presence of a strong inelastic scattering.

The adsorbate-adsorbate interaction mediated by Shockley surface states on, for example, the (111) faces of noble metals, yields an oscillatory form modulated by a 1/d 2 envelope at asymptotic adsorbate separations d . For this interaction we obtain a non-perturbative analytical estimate, specified by experimentally accessible Shockley-state parameters and the finite Fermi-level phase shift F 0, which characterize the standing-wave patterns observed in scanning tunnelling microscopy (STM) images. We provide explicit interaction results for the phase shift value F = - /2 suggested by the STM measurements of sulphur adsorbates on Cu(111), and we attempt to relate our results to the corresponding observations of correlations in the adsorbate distribution function.

We study the mechanism leading to the metallization of the $\beta$-SiC(001) Si-rich surface induced by hydrogen adsorption. We analyze the effects of band bending and demonstrate the existence of a quasi-2D electron gas, which originates from the donation of electrons from adsorbed hydrogen to bulk conduction states. We also provide a simple model that captures the main features of the results of first-principles calculations, and uncovers the basic physics of the process., Comment: accepted for publication in physica status solidi - Rapid Research Letters

We present three-dimensional numerical modeling results for gated Si/SiO2 quantum dot systems in the few-electron regime. In our simulations, the electrostatic confining potential results from the Poisson equation assuming a self-consistent Thomas-Fermi charge model. We find that a very thin SiO2 top insulating layer allows an effective control with single-electron confinement in quantum dots with radius less than 10nm and investigate the detailed potential and resulting charge densities. Our three-dimensional finite-element modeling tool allows future investigations of the charge coupling in gated few-electron quantum-dot cellular automata.

We predict in Si/Ge superlattices a dramatic high-temperature suppression of the perpendicular thermal transport. Total internal reflection confines the superlattice modes and significantly reduces the average group velocity at nonzero in-plane momenta. These consequences of the acoustic mismatch cause at high temperatures an order-of-magnitude reduction in the ratio of superlattice thermal conductivity to phonon relaxation time.

Large biomolecular systems, whose function may involve thousands of atoms, cannot easily be addressed with parameter-free density functional theory (DFT) calculations. Until recently a central problem was that such systems possess an inherent sparseness, that is, they are formed from components that are mutually separated by low-electron-density regions where dispersive forces contribute significantly to the cohesion and behavior. The introduction of, for example, the van der Waals density functional (vdW-DF) method [PRL 92, 246401 (2004)] has addressed part of this sparse-matter system challenge. However, while a vdW-DF study is often as computationally efficient as a study performed in the generalized gradient approximation, the scope of large-sparse-matter DFT is still limited by computer time and memory. It is costly to self-consistently determine the electron wavefunctions and hence the kinetic-energy repulsion. In this paper we propose and evaluate an adaption of the Harris scheme [PRB 31, 1770 (1985)]. This is done to speed up non-selfconsistent vdW-DF studies of molecular-system interaction energies. Also, the Harris-type analysis establishes a formal link between dispersion-interaction effects on the effective potential for electron dynamics and the impact of including selfconsistency in vdW-DF calculations [PRB 76, 125112 (2007)]., Comment: 15 pages