A recent paper [J. Chem. Phys. 132 134705 (2010)] illustrated the potential of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004)] for efficient first-principle accounts of structure and cohesion in molecular crystals. Since then, modifications of the original vdW-DF version (identified as vdW-DF1) has been proposed, and there is also a new version called vdW-DF2 [ArXiv 1003.5255], within the vdW-DF framework. Here we investigate the performance and nature of the modifications and the new version for the binding of a set of simple molecular crystals: hexamine, dodecahedrane, C60, and graphite. These extended systems provide benchmarks for computational methods dealing with sparse matter. We show that a previously documented enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based account close to and a few Å beyond binding separation persists in vdW-DF2. The calculation and analysis of the binding in molecular crystals requires appropriate computational tools. In this paper, we also present details on our real-space parallel implementation of the vdW-DF correlation and on the method used to generate asymptotic atom-based pair potentials based on vdW-DF.

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.

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We present density functional theory calculations of methanol molecular adsorption at the (0001) surface of α-Al 2 O 3 , for methanol coverages of 1/4 to 1 monolayer (ML). Adsorption energies, adsorption-induced restructuring of the surface, and induced changes to the electronic structure are calculated. We find that methanol bonds with its O atom to Al atoms at the α-Al 2 O 3 (0001) surface with an adsorption energy of 1.23 eV at coverage 1/4 ML, decreasing with coverage to 1.03 eV at 1 ML coverage. From calculations of the relaxed adsorption geometry and the angular dependence of the total energy, we predict an orientation of the adsorbed methanol molecule that has the molecular COH plane tilted away from the surface normal. The adsorption of methanol significantly restructures α-Al 2 O 3 (0001), especially for the outermost Al layer. Upon adsorption a small charge transfer from the molecule to the substrate takes place.

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We present density functional theory calculations of methanol and methoxy adsorption at the Cr-terminated α-Cr 2 O 3 (0001) surface. We report on the equilibrium geometries of the adsorbed methanol and methoxy molecules, analyse the bonding to the surface, and discuss the dissociation energetics of methanol into methoxy on the surface. We found that methanol adsorbs with its O atom situated above a threefold coordinated hollow site in the surface O layer at a distance of 2.12Å from the nearest-neighbour Cr atom, and with a calculated adsorption energy of 0.82 eV. For the methoxy molecule we found the optimum adsorption geometry to be with the methoxy O on top of a Cr atom and with the CO-axis tilted away from the surface normal by ∼55 • . Methoxy is strongly bound to the surface with an estimated adsorption energy of 3.3 eV.

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Adsorbing anthracene on a Cu͑111͒ surface results in a wide range of complex and intriguing superstructures spanning a coverage range from 1 per 17 to 1 per 15 substrate atoms. In accompanying first-principles density-functional theory calculations we show the essential role of van der Waals interactions in estimating the variation in anthracene adsorption energy and height across the sample. We can thereby evaluate the compression of the anthracene film in terms of continuum elastic properties, which results in an effective Young's modulus of 1.5 GPa and a Poisson ratio Ϸ0.1. These values suggest interpretation of the molecular monolayer as a porous material-in marked congruence with our microscopic observations.