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. In particular, a functional form, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401; Thonhauser et al 2007 Phys. Rev. B 76 125112), has been proposed for the nonlocal correlations between electrons and applied to various relevant molecules and materials, including to those layered systems like graphite, boron nitride and molybdenum sulfide, to dimers of benzene, polycyclic aromatic hydrocarbons (PAHs), doped benzene, cytosine and DNA base pairs, to nonbonding forces in molecules, to adsorbed molecules, like benzene, naphthalene, phenol and adenine on graphite, alumina and metals, to polymer and carbon nanotube (CNT) crystals, and hydrogen storage in graphite and metal-organic frameworks (MOFs), and to the structure of DNA and of DNA with intercalators. Comparison with results from wavefunction calculations for the smaller systems and with experimental data for the extended ones show the vdW-DF path to be promising. This could have great ramifications.

It is shown that it is now possible to include van der Waals interactions via a nonempirical implementation of density functional theory to describe the correlation energy in electronic structure calculations on infinite systems of no particular symmetry. The vdW-DF functional [Phys. Rev. Lett. 92, 246401 (2004)] is applied to the adsorption of benzene and naphthalene on an infinite sheet of graphite, as well as the binding between two graphite sheets. Comparison with recent thermal desorption data [Phys. Rev. B 69, 535406 (2004)] shows great promise for the vdW-DF method.A recent study of the interaction of polycyclic aromatic hydrocarbon molecules (PAH's) with the basal plane of graphite [1] provides experimental benchmark data that constitute an ideal challenge for our recently proposed density functional (vdW-DF) [2] which both includes van der Waals (vdW) interactions and all the benefits of the earlier state-of-the-art versions of the density-functional theory (DFT). Aiming at a better experimental characterization of the weak interlayer interactions in graphite, careful analysis of thermal-desorption kinetics yield activation energies for benzene and PAH's at submonolayer coverages with explicit error bars [1]. Our calculated values for the adsorption energy of benzene and naphthalene on graphene and for the weak interlayer interaction energy of graphene agree with the values deduced from experiment. From this we conclude that the vdW-DF is, indeed, very promising, and that it can be applied to systems that are neither periodic nor finite. This distinguishes it from the various wave-function methods that are often applied to vdW complexes.Our method differs also from a newly published study of adenine on graphite [3], which treats the vdW part of the correlation energy by a frequently used [4,5,6,7,8,9] semi-empirical method. This method introduces empirical damping functions applied to an asymptotic attractive 1/R 6 interaction assumed to occur between each pair of nuclei. At shorter distances this interaction is damped by a physically motivated, but arbitrary and varying functional form, which introduces one empirical parameter for every pair of atomic types in the complex. On the other hand our method (vdW-DF) for the correlation energy is completely free from empiricism, and although containing approximations, represents a firstprinciples density functional, which since the appearance of Ref. 2 is applicable to arbitrary geometries, and which is seamless as two fragments merge into a single one. As discussed later, it has been applied to a number of physical systems with promising results. The present application is particularly pertinent, however, as alternative first-principles methods for including vdW interactions are lacking for extended systems.Condensed matter is held together by several kinds of interatomic forces, including the ubiquitous vdW forces, which are particularly significant in sparse matter. For dense matter DFT has well proven its value, state-of-theart versions of it giving valu...

First-principles calculations of phenol adsorbed on two different surfaces, graphite͑0001͒ and ␣-Al 2 O 3 ͑0001͒, are performed with traditional semilocal density functional theory ͑DFT͒ and with a recently presented density functional ͑vdW-DF͒ that incorporates the dispersive van der Waals ͑vdW͒ interactions ͓Phys. Rev. Lett. 92, 246401 ͑2004͔͒. The vdW-DF is of decisive importance for describing the vdW bond of the phenol-graphite system and gives a secondary but not negligible vdW contribution for phenol on alumina. We find a predominantly covalent bond at the alumina surface. There, adsorption results in a binding separation ͑distance between surface Al and the O of the inclining phenol molecule͒ of 1.95 Å and a binding energy of 1.00 eV, evaluated within the generalized gradient approximation ͑GGA͒ of DFT, i.e., from covalency, with the energy increasing to around 1.2 eV when the contribution from vdW interactions is also accounted for. On graphite, with its pure vdW bond, the adsorption distance ͑separation between parallel surface and phenol molecule͒ is found to be 3.47 Å and the adsorption strength 0.56 eV. Comparison of the results for alumina and graphite mutually and with published results for nickel reveals significant differences in the adsorption of this model biomolecule.

No abstract

The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401). The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similarly sized naphthalene. On the basis of the single-molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption.