van der Waals forces in density functional theory: a review of the vdW-DF method.

A density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology is reviewed. The insights that led to the construction of the Rutgers-Chalmers van der Waals density functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasizing more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT.

van der Waals density functionals built upon the electron-gas tradition: facing the challenge of competing interactions.

The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K.

Benchmarking van der Waals density functionals with experimental data: potential-energy curves for H2 molecules on Cu(111), (100) and (110) surfaces.

Detailed physisorption data from experiment for the H(2) molecule on low-index Cu surfaces challenge theory. Recently, density functional theory (DFT) has been developed to account for nonlocal correlation effects, including van der Waals (dispersion) forces. We show that the functional vdW-DF2 gives a potential-energy curve, potential-well energy levels and difference in lateral corrugation promisingly close to the results obtained by resonant elastic backscattering-diffraction experiments.

Ab initio van der waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like.

The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations in the NVE ensemble using van der Waals (vdW) density-functional theory, i.e., using the new exchange-correlation functionals optPBE-vdW and vdW-DF2, where the latter has softer nonlocal correlation terms.

Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: a trend study of surface electronic factors.

This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMCs) in NaCl structure by means of density-functional theory calculations. The investigated substrates are the TM-terminated TMC(111) surfaces, of interest because of the presence of different types of surface resonances (SRs) on them and because of their technological importance in growth processes. Also, TM compounds have shown potential in catalysis applications.

From electronic structure to catalytic activity: a single descriptor for adsorption and reactivity on transition-metal carbides.

Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMC's) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SR's) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Brønsted-Evans-Polanyi relation and scaling relations apply.

Stacking interactions and DNA intercalation.

The relationship between stacking interactions and the intercalation of proflavine and ellipticine within DNA is investigated using a nonempirical van der Waals density functional for the correlation energy. Our results, employing a binary stack model, highlight fundamental, qualitative differences between base-pair-base-pair interactions and that of the stacked intercalator-base-pair system. The most notable result is the paucity of torque, which so distinctively defines the twist of DNA.

Density functional for van der Waals forces accounts for hydrogen bond in benchmark set of water hexamers.

A recent extensive study has investigated how various exchange-correlation (XC) functionals treat hydrogen bonds in water hexamers and has shown traditional generalized gradient approximation and hybrid functionals used in density-functional (DF) theory to give the wrong dissociation-energy trend of low-lying isomers and van der Waals (vdW) dispersion forces to give key contributions to the dissociation energy. The question raised whether functionals that incorporate vdW forces implicitly into the XC functional predict the correct lowest-energy structure for the water hexamer and yield accurate total dissociation energy is here answered affirmatively for the vdW-DF [M. Dion et al.

Density functional study of the adsorption and van der Waals binding of aromatic and conjugated compounds on the basal plane of MoS(2).

Accurate calculations of adsorption energies of cyclic molecules are of key importance in investigations of, e.g., hydrodesulfurization (HDS) catalysis.

A density functional for sparse matter.

Sparse matter is abundant and has both strong local bonds and weak nonbonding forces, in particular nonlocal van der Waals (vdW) forces between atoms separated by empty space. It encompasses a broad spectrum of systems, like soft matter, adsorption systems and biostructures. Density-functional theory (DFT), long since proven successful for dense matter, seems now to have come to a point, where useful extensions to sparse matter are available.

Stacking interactions and the twist of DNA.

The importance of stacking interactions for the Twist and stability of DNA is investigated using the fully ab initio van der Waals density functional (vdW-DF). Our results highlight the role that binary interactions between adjacent sets of base pairs play in defining the sequence-dependent Twists observed in high-resolution experiments. Furthermore, they demonstrate that additional stability gained by the presence of thymine is due to methyl interactions with neighboring bases, thus adding to our understanding of the mechanisms that contribute to the relative stability of DNA and RNA.

Application of van der Waals density functional to an extended system: adsorption of benzene and naphthalene on graphite.

It is shown that it is now possible to include van der Waals (vdW) interactions via a nonempirical implementation of density functional (DF) theory to describe the correlation energy in electronic structure calculations on infinite systems of no particular symmetry. The vdW-DF theory [Phys. Rev.

Potential--energy surfaces for excited states in extended systems.

With a simple and physically intuitive method, first-principles calculations of potential-energy surfaces are performed for excited states in a number of illustrative systems, including dimers (H(2) and NaCl) and gas-surface systems [Cl-Na(100) and Cl(2)-Na(100)]. It is based on density-functional theory and is a generalization of the Delta self-consistent field (DeltaSCF) method, where electron-hole pairs are introduced in order to model excited states, corresponding to internal electron transfers in the considered system. The desired excitations are identified by analysis of calculated electron orbitals, local densities of states, and charge densities.

van der Waals density functional for general geometries.

A scheme within density functional theory is proposed that provides a practical way to generalize to unrestricted geometries the method applied with some success to layered geometries [Phys. Rev. Lett.

Van der Waals density functional for layered structures.

To understand sparse systems, we must account for both strong local atom bonds and weak nonlocal van der Waals forces between atoms separated by empty space. A fully nonlocal functional form [Phys. Rev.

Self-organized one-dimensional electron systems on a low-symmetry oxide surface.

A new one-dimensional electron gas, metallic over a temperature range of 1-800 K, is predicted on the kappa-Al2O3(001;) surface by means of density-functional theory (DFT) calculations. The robustness against the Peierls instability is tested using a tight-binding model with DFT-calculated parameters. The critical transition temperature T(c) is shown to be smaller than 1 K.

Quantum origin of the oxygen storage capability of ceria.

The microscopic mechanism behind the extraordinary ability of ceria to store, release, and transport oxygen is explained on the basis of first-principles quantum mechanical simulations. The oxygen-vacancy formation energy in ceria is calculated for different local environments. The reversible CeO2-Ce2O3 reduction transition associated with oxygen-vacancy formation and migration is shown to be directly coupled with the quantum process of electron localization.

Stability of a flexible polar ionic crystal surface: metastable alumina and one-dimensional surface metallicity.

A first-principles study of kappa-Al2O3 (001) and (001-) reveals new features of ion-surface stability and electronic structure. The need to generalize Tasker's rules for surface stability of low-symmetry crystals is shown. Structurally, the presence of bulk tetrahedral Al ( Al(T)) causes giant surface relaxations, with O termination at (001).

Dissipative quantum dynamics in 2D: anisotropic dissipation and selective bond breaking in surface photochemistry.

The dissipative quantum dynamics of a model system, O2 at a Pt(111) surface, has been solved in two dimensions using a stochastic wave packet approach and parallel-computing techniques. It is found that, upon excitation, the dissipation anisotropy creates nonequilibrium and anisotropic energy storage between different reaction channels. The latter determines decisively the short-time reaction dynamics and, in particular, the branching ratio between desorption and dissociation, in agreement with recent experimental findings.

Nature, strength, and consequences of indirect adsorbate interactions on metals.

Atoms and molecules adsorbed on metals affect each other indirectly even over considerable distances. Via systematic density-functional calculations, we establish the nature and strength of such interactions, and explain for what adsorbate systems they critically affect important materials properties. This is verified in kinetic Monte Carlo simulations of epitaxial growth, which help rationalize a number of recent experimental reports on anomalously low diffusion prefactors.

Nature of metal-ceramic adhesion: computational experiments with Co on TiC.

By means of first-principles computational experiments, the microscopic mechanism of metal-carbide adhesion is revealed. Density-functional-theory results for the Co /TiC(001) interface show the interface bonding to be dominated by Co-C bonds. The effective number of bonds is controlled by an interplay between lattice mismatch and relaxation effects.