MBD + C: How to Incorporate Metallic Character into Atom-Based Dispersion Energy Schemes.

The dispersion component of the van der Waals interaction in low-dimensional metals is known to exhibit anomalous "Type-C non-additivity" [ 1157]. This causes dispersion energy behavior at asymptotically large separations that is missed by popular atom-based schemes for dispersion energy calculations. For example, the dispersion interaction energy between parallel metallic nanotubes at separation falls off asymptotically as approximately , whereas current atom-based schemes predict asymptotically.

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science.

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

Exact Sum-Rule Approach to Polarizability and Asymptotic van der Waals Functionals─Derivation of Exact Single-Particle Benchmarks.

Using a sum-rule approach, we develop an exact theoretical framework for polarizability and asymptotic van der Waals correlation energy functionals of small isolated objects. The functionals require only monomer ground-state properties as input. Functional evaluation proceeds via solution of a single position-space differential equation, without the usual summations over excited states or frequency integrations.

Faraday cage screening reveals intrinsic aspects of the van der Waals attraction.

General properties of the recently observed screening of the van der Waals (vdW) attraction between a silica substrate and silica tip by insertion of graphene are predicted using basic theory and first-principles calculations. Results are then focused on possible practical applications, as well as an understanding of the nature of vdW attraction, considering recent discoveries showing it competing against covalent and ionic bonding. The traditional view of the vdW attraction as arising from pairwise-additive London dispersion forces is considered using Grimme's "D3" method, comparing results to those from Tkatchenko's more general many-body dispersion (MBD) approach, all interpreted in terms of Dobson's general dispersion framework.

Casimir-Polder Size Consistency: A Constraint Violated by Some Dispersion Theories.

A key goal in quantum chemistry methods, whether ab initio or otherwise, is to achieve size consistency. In this work we formulate the related idea of "Casimir-Polder size consistency" that manifests in long-range dispersion energetics. We show that local approximations in time-dependent density functional theory dispersion energy calculations violate the consistency condition because of incorrect treatment of highly nonlocal "xc kernel" physics, by up to 10% in our tests on closed-shell atoms.

Spooky correlations and unusual van der Waals forces between gapless and near-gapless molecules.

We consider the zero-temperature van der Waals (vdW) interaction between two molecules, each of which has a zero or near-zero electronic gap between a ground state and the first excited state, using a toy model molecule (equilateral H) as an example. We show that the van der Waals energy between two ground state molecules falls off as D instead of the usual D dependence, when the molecules are separated by distance D. We show that this is caused by a perfect "spooky" correlation between the two fluctuating electric dipoles.

Binding and interlayer force in the near-contact region of two graphite slabs: experiment and theory.

Via a novel experiment, Liu et al. [Phys. Rev.

Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3.

The organic-inorganic hybrid perovskite CH3NH3PbI3 is a novel light harvester, which can greatly improve the solar-conversion efficiency of dye-sensitized solar cells. In this article, a first-principle theoretical study is performed using local, semi-local and non-local exchange-correlation approximations to find a suitable method for this material. Our results, using the non-local optB86b + vdWDF functional, excellently agree with the experimental data.

Dispersion corrections in graphenic systems: a simple and effective model of binding.

We combine high-level theoretical and ab initio understanding of graphite to develop a simple, parametrized force-field model of interlayer binding in graphite, including the difficult non-pairwise-additive coupled-fluctuation dispersion interactions. The model is given as a simple additive correction to standard density functional theory (DFT) calculations, of form ΔU(D) = f(D)[U(vdW)(D) - U(DFT)(D)] where D is the interlayer distance. The functions are parametrized by matching contact properties, and long-range dispersion to known values, and the model is found to accurately match high-level ab initio results for graphite across a wide range of D values.

Assessment of range-separated time-dependent density-functional theory for calculating C6 dispersion coefficients.

We assess a variant of linear-response range-separated time-dependent density-functional theory (TDDFT), combining a long-range Hartree-Fock (HF) exchange kernel with a short-range adiabatic exchange-correlation kernel in the local-density approximation (LDA) for calculating isotropic C6 dispersion coefficients of homodimers of a number of closed-shell atoms and small molecules. This range-separated TDDFT tends to give underestimated C6 coefficients of small molecules with a mean absolute percentage error of about 5%, a slight improvement over standard TDDFT in the adiabatic LDA which tends to overestimate them with a mean absolute percentage error of 8%, but close to time-dependent Hartree-Fock which has a mean absolute percentage error of about 6%. These results thus show that introduction of long-range HF exchange in TDDFT has a small but beneficial impact on the values of C6 coefficients.

Electron affinities and ionisation potentials for atoms via "benchmark" tdDFT calculations with and without exchange kernels.

One of the known weaknesses of the adiabatic connection fluctuation dissipation (ACFD) correlation energy functional under the direct random-phase approximation (RPA) is its failure to accurately predict energy differences between dissimilar systems. In this work we evaluate ionisation potentials I and electron affinities A for atoms and ions with one to eighteen electrons using the ACFD functional under the RPA, and with the "PGG (Petersilka-Gossmann-Gross)" and "RXH (radial exchange hole)" model exchange kernels. All calculations are carried out using a real-space, all electron method with an exact exchange groundstate to minimise errors.

The flexible nature of exchange, correlation, and Hartree physics: resolving "delocalization" errors in a "correlation free" density functional.

By exploiting freedoms in the definitions of "correlation," "exchange," and "Hartree" physics in ensemble systems, we better generalise the notion of "exact exchange" (EXX) to systems with fractional occupations of the frontier orbitals, arising in the dissociation limit of some molecules. We introduce the linear EXX ("LEXX") theory whose pair distribution and energy are explicitly piecewise linear in the occupations f(i)(σ). We provide explicit expressions for these functions for frontier s and p shells.

Shear induced formation of carbon and boron nitride nano-scrolls.

A 'top down' synthesis of carbon and hexagonal boron nitride (h-BN) nano-scrolls has been developed using the shear forces within dynamic thin films of N-methyl-2-pyrrolidone (NMP) generated on a rapidly rotating spinning disc processor (SDP), along with a theoretical understanding of the formation of the scrolls.

Quantum continuum mechanics made simple.

In this paper we further explore and develop the quantum continuum mechanics (QCM) of Tao et al. [Phys. Rev.

Vortex fluidic exfoliation of graphite and boron nitride.

Graphite is exfoliated into graphene by shearing vortex fluidic films of N-methyl-pyrrolidone (NMP), as a controlled process for preparing oxide free graphene, and for exfoliating the corresponding boron nitride sheets.

Calculation of dispersion energies.

We summarize the theory of van der Waals (dispersion) forces, with emphasis on recent microscopic approaches that permit the prediction of forces between solids and nanostructures right down to intimate contact and binding. Some connections are pointed out between microscopic theory and macroscopic Lifshitz theory.

Dispersion interaction in hydrogen-chain models.

We have investigated the dispersion interaction in hydrogen chain models via density functional theory-based symmetry-adapted perturbation theory using the asymptotically corrected PBE0 energy functional. The quasimetallic and the insulating prototype systems were chosen to be hydrogen chains with equally and alternately spaced H(2) units, respectively. The dependence of the dispersion energy on the chain length for quasimetallic and insulating cases has been determined for two chains arranged either in pointing or in parallel geometries.

Inhomogeneous STLS theory and TDCDFT.

We consider the inhomogeneous Singwi-Tosi-Land-Sjolander theory (ISTLS) of Dobson, Wang and Gould (Phys. Rev. B, 2002, 66, 081108(R)), and possible generalizations, in the context of time dependent current density functional theory (TDCDFT) and its tensor exchange correlation kernel fxc.

Asymptotics of the dispersion interaction: analytic benchmarks for van der Waals energy functionals.

We show that the usual sum of R-6 contributions from elements separated by distance R can give qualitatively wrong results for the electromagnetically nonretarded van der Waals interaction between nonoverlapping bodies. This occurs for anisotropic nanostructures that have a zero electronic energy gap, such as metallic nanotubes or nanowires, and nanolayered systems including metals and graphene planes. In all these cases our analytic microscopic calculations give an interaction falling off with a power of separation different from the conventional value.