Interpretation of van der Waals density functionals

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.Comment: 13 pages, 6 figures. Submited to Physical Review

Extent of Fock-exchange mixing for a hybrid van der Waals density functional?

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.Comment: 18 pages, 5 figures, accepted by J. Chem. Phy

Signatures of van der Waals binding: a coupling-constant scaling analysis

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.Comment: 15 pages, 8 figure

Van der Waals interactions of parallel and concentric nanotubes

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.Comment: 6 pages, 4 figures (5 figure files

The van der Waals Interactions of Concentric Nanotubes

Concentric nanotubes are stabilized by a competition between the short-range repulsion and the long-range van der Waals binding. At the relevant binding distances (3.4 Ångström) we find that traditional first-principle density-functional theory (DFT) calculations can only par- tially account for these combined interactions and that an accurate quantum-physics account of structure and dynamics must also include a calculation of the van der Waals forces. We use a successful model of the electrodynamical response of the nanotube electron gas to provide such a combined description in which we reflect the important self-consistent screening effects arising within the nanotube electron gases. Our description differs significantly from traditional asymptotic calculations of the van der Waals interactions. Work supported by the Carl Tryggers Foundation, W. and M. Lundgren Foundation, the Swedish Research Council (VR), and the Swedish Foundation for Strategic Research (SSF)

Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results

The dispersive interaction between nanotubes is investigated through ab initio theory calculations and in an analytical approximation. A van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to determine and compare the binding of a pair of nanotubes as well as in a nanotube crystal. To analyze the interaction and determine the importance of morphology, we furthermore compare results of our ab initio calculations with a simple analytical result that we obtain for a pair of well-separated nanotubes. In contrast to traditional density functional theory calculations, the vdW-DF study predicts an intertube vdW bonding with a strength that is consistent with recent observations for the interlayer binding in graphitics. It also produce a nanotube wall-to-wall separation which is in very good agreement with experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal binding energy can be approximated by a sum of nanotube-pair interactions when these are calculated in vdW-DF. This observation suggests a framework for an efficient implementation of quantum-physical modeling of the CNT bundling in more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure

Oxidation of Mg(0001): A combined Experimental and Theoretical Study

The oxidation of magnesium proceeds in several stages, beginning with the oxygen dissociation process and ending with the formation of magnesium oxides. Our focus is on the intermediate oxidation state at the Mg(0001) surface, whose geometrical structure is an unsettled problem. By combining the results of high-accuracy electronic structure calculations with angle-scanned x-ray photoelectron diffraction measurements we are able to unambiguously determine the structure of the Mg(0001) surface upon oxidation. In contrast to previous studies of Mg(0001) oxidation and unlike the case of aluminum oxidation we find a rather unanticipated surface oxide structure, consisting of two mixed oxygen-magnesium layers on top of an almost undisturbed Mg(0001) surface. This unusual surface oxide structure is locally formed already at very low coverages of 0.1 monolayer and grows laterally with increasing oxygen coverage up to 2 monolayers

A van der Waals density functional study of chloroform and other trihalomethanes on graphene

A computational study of chloroform (CHCl3) and other trihalomethanes (THMs) adsorbed on graphene is presented. The study uses the van der Waals density functional method to obtain ad- sorption energies and adsorption structures for these molecules of environmental concern. In this study, chloroform is found to adsorb with the H atom pointing away from graphene, with adsorption energy 357 meV (34.4 kJ/mol). For the other THMs studied the calculated adsorption energy values vary from 206 meV (19.9 kJ/mol) for fluoroform (CHF3) to 404 meV (39.0 kJ/mol) for bromoform (CHBr3). The corrugation of graphene as seen by the THMs is small, the difference in adsorption energy along the graphene plane is less than 6 meV for chloroform

Towards a working density-functional theory for polymers: First-principles determination of the polyethylene crystal structure

Equilibrium polyethylene crystal structure, cohesive energy, and elastic constants are calculated by density-functional theory applied with a recently proposed density functional (vdW-DF) for general geometries [Phys. Rev. Lett. 92, 246401 (2004)] and with a pseudopotential-planewave scheme. The vdW-DF with its account for the long-ranged van der Waals interactions gives not only a stabilized crystal structure but also values of the calculated lattice parameters and elastic constants in quite good agreement with experimental data, giving promise for successful application to a wider range of polymers.Comment: 4 pages, 3 figure