van der Waals density functional calculations of binding in molecular crystals

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 A, 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.Comment: 5 pages, 4 figure

Effective elastic properties of a van der Waals molecular monolayer at a metal surface

Adsorbing anthracene on a Cu(111) 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\u27s modulus of 1.5 GPa and a Poisson ratio approximate to 0.1. These values suggest interpretation of the molecular monolayer as a porous material-in marked congruence with our microscopic observations

Adsorption of organic molecules at sufaces: A first principles investigation

The adsorption of a molecule at a surface is a fundamental step in a wide variety of industrially relevant phenomena, including adhesion, corrosion, and catalysis. The work presented in this thesis is motivated by the desire to contribute to a better understanding of the factors affecting the adhesion between an organic coating/adhesive and an aluminium alloy surface. A key factor is the nature and strength of the interfacial bonds between the binder polymers of the organic coating/adhesive and the substrate. The size of the polymers and complexity of the polymer-substrate interactions preclude a detailed, atomic-level description. The strategy followed in this thesis is to study the adsorption of small organic molecules, representing fragments of the industrially relevant amine-cured epoxides, with various surfaces, of metal oxides (α-Al2O3(0001) and α-Cr2O3(0001)), bimetallic alloys (NiAl(110)), and graphite(0001). This thesis consists of two parts, an introductory text and a collection of five papers. In the included papers we present results from density functional theory (DFT) calculations on the adsorption of methanol and methylamine on α-Al2O3(0001) and α-Cr2O3(0001), phenol on α-Al2O3(0001) and graphite(0001), and methoxy on α-Cr2O3(0001) and NiAl(110). We describe in detail the adsorption sites and geometry, and the nature and strength of the bonding at these surfaces. The majority of adsorption systems considered in this thesis are well described by traditional implementations of DFT. However, the adsorption of phenol on graphite is predominantly governed by van der Waals interactions. These interactions requires approximations beyond traditional DFT. In this thesis a recently presented functional (vdW-DF) is employed, and is found to be of decisive importance for describing the phenol-graphite interactions. We calculate the contribution from vdW interactions to the adsorption of phenol on α-Al2O3(0001), and compare their contribution to the adsorption bond to other forces

Children's understanding of quantity and their ability to use graphical information

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Adsorption energies <i>E</i>_<i>a</i> for toluene on graphene.

<p>Calculated with various values of parameters, of real-space grid points (gpts), number of Brillouin zone k-points (kpts), and exponent of energy convergence threshold <i>n</i>, 1.5 ⋅ 10^−<i>n</i> eV/electron. The student research project provided the “student” data for toluene, whereas the medium- and high-quality toluene data were obtained later by the research group [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0159168#pone.0159168.ref002" target="_blank">2</a>]. As a comparison, results of experimental measurements from the literature [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0159168#pone.0159168.ref020" target="_blank">20</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0159168#pone.0159168.ref021" target="_blank">21</a>] are also shown, however, these results cannot be directly compared, as discussed in the main text.</p