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See detailRole of Dispersion Interactions in the Polymorphism and Entropic Stabilization of the Aspirin Crystal
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in PHYSICAL REVIEW LETTERS (2014), 113(5),

Aspirin has been used and studied for over a century but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate ... [more ▼]

Aspirin has been used and studied for over a century but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable. [less ▲]

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See detailModeling Adsorption and Reactions of Organic Molecules at Metal Surfaces
Liu, Wei; Tkatchenko, Alexandre UL; Scheffler, Matthias

in ACCOUNTS OF CHEMICAL RESEARCH (2014), 47(11, SI), 3369-3377

CONSPECTUS: The understanding of adsorption and reactions of (large) organic molecules at metal surfaces plays an increasingly important role in modern surface science and technology. Such hybrid ... [more ▼]

CONSPECTUS: The understanding of adsorption and reactions of (large) organic molecules at metal surfaces plays an increasingly important role in modern surface science and technology. Such hybrid inorganic/organic systems (HIOS) are relevant for many applications in catalysis, light emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. Obviously, the predictive modeling and understanding of the structure and stability of such hybrid systems is an essential prerequisite for tuning their electronic properties and functions. At present, density-functional theory (DFT) is the most promising approach to study the structure, stability, and electronic properties of complex systems, because it can be applied to both molecules and solids comprising thousands of atoms. However state-of-the-art approximations to DFT do not provide a consistent and reliable description for HIOS, which is largely due to two issues: (i) the self-interaction of the electrons with themselves arising from the Hartree term of the total energy that is not fully compensated in approximate exchange-correlation functionals, and (ii) the lack of long-range part of the ubiquitous van der Waals (vdW) interactions. The self-interaction errors sometimes lead to incorrect description of charge transfer and electronic level alignment in HIOS, although for molecules adsorbed on metals these effects will often cancel out in total energy differences. Regarding vdW interactions, several promising vdW-inclusive DFT-based methods have been recently demonstrated to yield remarkable accuracy for intermolecular interactions in the gas phase. However, the majority of these approaches neglect the nonlocal collective electron response in the vdW energy tail, an effect that is particularly strong in condensed phases and at interfaces between different materials. Here we show that the recently developed DFT+vdW(surf) method that accurately accounts for the collective electronic response effects enables reliable modeling of structure and stability for a broad class of organic molecules adsorbed on metal surfaces. This method was demonstrated to achieve quantitative accuracy for aromatic hydrocarbons (benzene, naphthalene, anthracene, and diindenoperylene), C-60, and sulfur/oxygen-containing molecules (thiophene, NTCDA, and PTCDA) on dose-packed and stepped metal surfaces, leading to an overall accuracy of 0.1 angstrom in adsorption heights and 0.1 eV in binding energies with respect to state-of-the-art experiments. An unexpected finding is that vdW interactions contribute more to the binding of strongly bound molecules on transition-metal surfaces than for molecules physisorbed on coinage metals. The accurate inclusion of vdW interactions also significantly improves tilting angles and adsorption heights for all the studied molecules, and can qualitatively change the potential-energy surface for adsorbed molecules with flexible functional groups. Activation barriers for molecular switches and reaction precursors are modified as well. [less ▲]

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See detailSize effects in the interface level alignment of dye-sensitized TiO 2 clusters
Marom, N.; Körzdörfer, T.; Ren, X. et al

in Journal of Physical Chemistry Letters (2014), 5(14), 2395-2401

The efficiency of dye-sensitized solar cells (DSCs) depends critically on the electronic structure of the interfaces in the active region. We employ recently developed dispersion-inclusive density ... [more ▼]

The efficiency of dye-sensitized solar cells (DSCs) depends critically on the electronic structure of the interfaces in the active region. We employ recently developed dispersion-inclusive density functional theory (DFT) and GW methods to study the electronic structure of TiO2 clusters sensitized with catechol molecules. We show that the energy level alignment at the dye-TiO2 interface is the result of an intricate interplay of quantum size effects and dynamic screening effects and that it may be manipulated by nanostructuring and functionalizing the TiO2. We demonstrate that the energy difference between the catechol LUMO and the TiO2 LUMO, which is associated with the injection loss in DSCs, may be reduced significantly by reducing the dimensions of nanostructured TiO2 and by functionalizing the TiO2 with wide-gap moieties, which contribute additional screening but do not interact strongly with the frontier orbitals of the TiO2 and the dye. Precise control of the electronic structure may be achieved via "interface engineering" in functional nanostructures. © 2014 American Chemical Society. [less ▲]

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See detailNon-additivity of molecule-surface van der Waals potentials from force measurements
Wagner, Christian; Fournier, Norman; Ruiz, Victor G. et al

in NATURE COMMUNICATIONS (2014), 5

Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system ... [more ▼]

Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction. [less ▲]

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See detailvan der Waals Interactions Determine Selectivity in Catalysis by Metallic Gold
Rodriguez-Reyes, Juan Carlos F.; Siler, Cassandra G. F.; Liu, Wei et al

in JOURNAL OF THE AMERICAN CHEMICAL SOCIETY (2014), 136(38), 13333-13340

To achieve high selectivity for catalytic reactions between two or more reactants on a heterogeneous catalyst, the relative concentrations of the reactive intermediates on the surface must be optimized ... [more ▼]

To achieve high selectivity for catalytic reactions between two or more reactants on a heterogeneous catalyst, the relative concentrations of the reactive intermediates on the surface must be optimized. If species compete for binding sites, their concentrations depend on their relative binding strengths to the surface. In this article we describe a general framework for predicting the relative stability of organic intermediates involved in oxygen-assisted reactions on metallic gold with broad relevance to catalysis by metals. Combining theory and experiment, we establish that van der Waals interactions between the reactive intermediates and the surface, although weak, determine relative stabilities and thereby dictate the conditions for optimum selectivity. The inclusion of these interactions is essential for predicting these trends. The concepts and methods employed here have broad applicability for determining the stability of intermediates on the surfaces of catalytic metals and specifically demonstrate the critical role of weak interactions in determining reaction selectivity among reactions of complex molecules. [less ▲]

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See detailHigh-Throughput Investigation of the Geometry and Electronic Structures of Gas-Phase and Crystalline Polycyclic Aromatic Hydrocarbons
Schatschneider, Bohdan; Monaco, Stephen; Liang, Jian-Jie et al

in JOURNAL OF PHYSICAL CHEMISTRY C (2014), 118(34), 19964-19974

The quest for cheap, light, flexible materials for use in electronics applications has resulted in the exploration of soft organic materials as possible candidates, and several polycyclic aromatic ... [more ▼]

The quest for cheap, light, flexible materials for use in electronics applications has resulted in the exploration of soft organic materials as possible candidates, and several polycyclic aromatic hydrocarbons (PAHs) have been shown to be versatile (semi)conductors. In this investigation, dispersion inclusive density functional theory is used to explore all of the current crystalline PAHs within the Cambridge Structure Database (CSD) from both structural and electronic standpoints. Agreement is achieved between the experimental and calculated crystalline structures, as well as the electronic properties. Specifically, variation between the mass densities, unit cell parameters, and intermolecular close contact fractions were within +5\% +/-2\%, and +/-1 of experiment, respectively. It is found that a simple addition of a similar to 1 eV constant to the DFT-PBE gaps provides good agreement with the experimental optical gaps of both gas phase (within +/-2.6\%) and crystalline (within +/-3.5\%) PAHs. Structural and electronic analysis revealed several correlations/trends where ultimately limits in the band gaps as a function of structure are established. Finally, analysis of the difference between band gaps of the isolated molecules and crystals (Delta E-g(Xtal-Mols)) demonstrates that Delta E-g(Xtal-Mols) can be captured qualitatively by PBE and PBE0 functionals, yet significant quantitative deviations remain between these functionals and experiment. [less ▲]

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See detailMany-body van der Waals interactions in molecules and condensed matter
Distasio, R. A.; Gobre, V. V.; Tkatchenko, Alexandre UL

in Journal of Physics Condensed Matter (2014), 26(21),

This work reviews the increasing evidence that many-body van der Waals (vdW) or dispersion interactions play a crucial role in the structure, stability and function of a wide variety of systems in biology ... [more ▼]

This work reviews the increasing evidence that many-body van der Waals (vdW) or dispersion interactions play a crucial role in the structure, stability and function of a wide variety of systems in biology, chemistry and physics. Starting with the exact expression for the electron correlation energy provided by the adiabatic connection fluctuation-dissipation theorem, we derive both pairwise and many-body interatomic methods for computing the long-range dispersion energy by considering a model system of coupled quantum harmonic oscillators within the random-phase approximation. By coupling this approach to density functional theory, the resulting many-body dispersion (MBD) method provides an accurate and efficient scheme for computing the frequency-dependent polarizability and many-body vdW energy in molecules and materials with a finite electronic gap. A select collection of applications are presented that ascertain the fundamental importance of these non-bonded interactions across the spectrum of intermolecular (the S22 and S66 benchmark databases), intramolecular (conformational energies of alanine tetrapeptide) and supramolecular (binding energy of the 'buckyball catcher') complexes, as well as molecular crystals (cohesive energies in oligoacenes). These applications demonstrate that electrodynamic response screening and beyond-pairwise many-body vdW interactions - both captured at the MBD level of theory - play a quantitative, and sometimes even qualitative, role in describing the properties considered herein. This work is then concluded with an in-depth discussion of the challenges that remain in the future development of reliable (accurate and efficient) methods for treating many-body vdW interactions in complex materials and provides a roadmap for navigating many of the research avenues that are yet to be explored. © 2014 IOP Publishing Ltd. [less ▲]

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See detailInteraction of Isophorone with Pd(111): A Combination of Infrared Reflection-Absorption Spectroscopy, Near-Edge X-ray Absorption Fine Structure, and Density Functional Theory Studies
Dostert, Karl-Heinz; O'Brien, Casey P.; Riedel, Wiebke et al

in JOURNAL OF PHYSICAL CHEMISTRY C (2014), 118(48), 27833-27842

Atomistic level understanding of interaction of alpha,beta-unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired ... [more ▼]

Atomistic level understanding of interaction of alpha,beta-unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired selectivity toward C-C or C-O bond hydrogenation. The interaction of this class of compounds with transition metals was investigated on alpha,beta-unsaturated ketone isophorone on Pd(111) as a prototypical system. In this study, infrared reflectionabsorption spectroscopy (IRAS), near-edge X-ray absorption fine structure (NEXAFS) experiments and density functional theory calculations including van der Waals interactions (DFT+vdW) were combined to obtain detailed information on the binding of isophorone to palladium at different coverages and on the effect of preadsorbed hydrogen on the binding and adsorption geometry. According to these experimental observations and the results of theoretical calculations, isophorone adsorbs on Pd(111) in a flat-lying geometry at low coverages. With increasing coverage, both C-C and C-O bonds of isophorone tilt with respect to the surface plane. The tilting is considerably more pronounced for the C-C bond on the pristine Pd(111) surface, indicating a prominent perturbation and structural distortion of the conjugated p system upon interaction with Pd. Preadsorbed hydrogen leads to higher tilting angles of both p bonds, which points to much weaker interaction of isophorone with hydrogen-precovered Pd and suggests the conservation of the in-plane geometry of the conjugated pi system. The results of the DFT+vdW calculations provide further insights into the perturbation of the molecular structure of isophorone on Pd(111). [less ▲]

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See detailUnderstanding the Structure and Electronic Properties of Molecular Crystals Under Pressure: Application of Dispersion Corrected DFT to Oligoacenes
Schatschneider, Bohdan; Monaco, Stephen; Tkatchenko, Alexandre UL et al

in JOURNAL OF PHYSICAL CHEMISTRY A (2013), 117(34), 8323-8331

Oligoacenes form a fundamental class of polycyclic aromatic hydrocarbons (PAR) which have been extensively explored for use as organic (semi) conductors in the bulk phase and thin films. For this reason ... [more ▼]

Oligoacenes form a fundamental class of polycyclic aromatic hydrocarbons (PAR) which have been extensively explored for use as organic (semi) conductors in the bulk phase and thin films. For this reason it is important to understand their electronic properties in the condensed phase. In this investigation, we use density functional theory with Tkatchenko-Scheffler dispersion correction to explore several crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) under pressures up to 25 GPa in an effort to uncover unique electronic/optical properties. Excellent agreement with experiment is achieved for the pressure dependence of the crystal structure unit cell parameters, densities, and intermolecular close contacts. The pressure dependence of the band gaps is investigated as well as the pressure induced phase transition of tetracene using both generalized gradient approximated and hybrid functionals. It is concluded that none of the oligoacenes investigated become conducting under elevated pressures assuming that the molecular identity of the system is maintained. [less ▲]

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See detailSeamless and Accurate Modeling of Organic Molecular Materials
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in Journal of Physical Chemistry Letters (2013), 4(6), 1028-1033

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der ... [more ▼]

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der Waals or dispersion interactions, but few go beyond a pairwise additive description of dispersion, ignoring its many-body nature. Here we use two databases to show how a many-body approach to dispersion can seamlessly model both solid and gas-phase interactions within the coveted ``chemical accuracy'' benchmark, while the underlying pairwise approach fails for solid-state interactions due to the absence of many-body polarization and energy contributions. Our results show that recently developed methods that treat the truly collective nature of dispersion interactions are able to reach the accuracy required for predicting molecular materials, when coupled with nonempirical density functionals. [less ▲]

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See detailRole of methyl-induced polarization in ion binding
Rossi, Mariana; Tkatchenko, Alexandre UL; Rempe, Susan B. et al

in Proceedings of the National Academy of Sciences of the United States of America (2013), 110(32), 12978-12983

The chemical property of methyl groups that renders them indispensable to biomolecules is their hydrophobicity. Quantum mechanical studies undertaken here to understand the effect of point substitutions ... [more ▼]

The chemical property of methyl groups that renders them indispensable to biomolecules is their hydrophobicity. Quantum mechanical studies undertaken here to understand the effect of point substitutions on potassium (K-) channels illustrate quantitatively how methyl-induced polarization also contributes to biomolecular function. K- channels regulate transmembrane salt concentration gradients by transporting K+ ions selectively. One of the K+ binding sites in the channel's selectivity filter, the S4 site, also binds Ba2+ ions, which blocks K+ transport. This inhibitory property of Ba2+ ions has been vital in understanding K-channel mechanism. In most K-channels, the S4 site is composed of four threonine amino acids. The K channels that carry serine instead of threonine are significantly less susceptible to Ba2+ block and have reduced stabilities. We find that these differences can be explained by the lower polarizability of serine compared with threonine because serine carries one less branched methyl group than threonine. A T -> S substitution in the S4 site reduces its polarizability, which, in turn, reduces ion binding by several kilocalories per mole. Although the loss in binding affinity is high for Ba2+, the loss in K+ binding affinity is also significant thermodynamically, which reduces channel stability. These results highlight, in general, how biomolecular function can rely on the polarization induced by methyl groups especially those that are proximal to charged moieties, including ions titratable amino acids, sulfates, phosphates, and nucleotides. [less ▲]

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See detailMolecular switches from benzene derivatives adsorbed on metal surfaces
Liu, W.; Filimonov, S. N.; Carrasco, J. et al

in Nature Communications (2013), 4

Transient precursor states are often experimentally observed for molecules adsorbing on surfaces. However, such precursor states are typically rather short-lived, quickly yielding to more stable ... [more ▼]

Transient precursor states are often experimentally observed for molecules adsorbing on surfaces. However, such precursor states are typically rather short-lived, quickly yielding to more stable adsorption configurations. Here we employ first-principles calculations to systematically explore the interaction mechanism for benzene derivatives on metal surfaces, enabling us to selectively tune the stability and the barrier between two metastable adsorption states. In particular, in the case of the tetrachloropyrazine molecule, two equally stable adsorption states are identified with a moderate and conceivably reversible barrier between them. We address the feasibility of experimentally detecting the predicted bistable behaviour and discuss its potential usefulness in a molecular switch. © 2013 Macmillan Publishers Limited. All rights reserved. [less ▲]

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See detailUnderstanding Structure and Bonding of Multilayered Metal-Organic Nanostructures
Egger, David A.; Ruiz, Victor G.; Said, Wissam A. et al

in JOURNAL OF PHYSICAL CHEMISTRY C (2013), 117(6), 3055-3061

For organic and hybrid electronic devices, the physicochemical properties of the contained interfaces play a dominant role. To disentangle the various interactions occurring at such heterointerfaces we ... [more ▼]

For organic and hybrid electronic devices, the physicochemical properties of the contained interfaces play a dominant role. To disentangle the various interactions occurring at such heterointerfaces we here model a complex, yet prototypical, three-component system consisting of a Cu-phthalocyanine (CuPc) film on a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) monolayer adsorbed on Ag(111). The two encountered interfaces are similar, as in both cases there would be no bonding without van der Waals interactions. Still they are also distinctly different, as only at the Ag-(111) PTCDA interface do massive charge-rearrangements occur. Using recently developed theoretical tools, we show that it has become possible to provide atomistic insight into the physical and chemical processes in this comparatively complex nanostructure distinguishing between interactions involving local rearrangements of the charge density and long-range van der Waals attraction. [less ▲]

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See detailNoncovalent Interactions of DNA Bases with Naphthalene and Graphene
Cho, Yeonchoo; Min, Seung Kyu; Yun, Jeonghun et al

in JOURNAL OF CHEMICAL THEORY AND COMPUTATION (2013), 9(4), 2090-2096

The complexes of a DNA base bound to graphitic systems are studied. Considering naphthalene as the simplest graphitic system, DNA base naphthalene complexes are scrutinized at high levels of ab initio ... [more ▼]

The complexes of a DNA base bound to graphitic systems are studied. Considering naphthalene as the simplest graphitic system, DNA base naphthalene complexes are scrutinized at high levels of ab initio theory including coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] at the complete basis set (CBS) limit. The stacked configurations are the most stable, where the CCSD(T)/CBS binding energies of guanine, adenine, thymine, and cytosine are 9.31 8.48, 8.53, 7.30 kcal/mol, respectively. The energy components are investigated using symmetry-adapted perturbation theory based on density functional theory including the dispersion energy. We compared the CCSD(T)/CBS results with several density functional methods applicable to periodic systems. Considering accuracy and availability, the optB86b nonlocal functional and the Tkatchenko-Scheffler functional are used to study the binding energies of nucleobases on graphene. The predicted values are 18-24 kcal/mol, though many-body effects on screening and energy need to be further considered. [less ▲]

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See detailMany-Body Dispersion Interactions in Molecular Crystal Polymorphism
Marom, Noa; DiStasio, Jr; Atalla, Viktor et al

in ANGEWANDTE CHEMIE-INTERNATIONAL EDITION (2013), 52(26), 6629-6632

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See detailAssessment and Validation of Machine Learning Methods for Predicting Molecular Atomization Energies
Hansen, Katja; Montavon, Gregoire; Biegler, Franziska et al

in JOURNAL OF CHEMICAL THEORY AND COMPUTATION (2013), 9(8), 3404-3419

The accurate and reliable prediction of properties of molecules typically requires computationally intensive quantum-chemical calculations. Recently, machine learning techniques applied to ab initio ... [more ▼]

The accurate and reliable prediction of properties of molecules typically requires computationally intensive quantum-chemical calculations. Recently, machine learning techniques applied to ab initio calculations have been proposed as an efficient approach for describing the energies of molecules in their given ground-state structure throughout chemical compound space (Rupp et al. Phys. Rev. Lett. 2012 108, 058301). In this paper we outline a number of established machine learning techniques and investigate the influence of the molecular representation on the methods performance. The best methods achieve prediction errors of 3 kcal/mol for the atomization energies of a wide variety of molecules. Rationales for this performance improvement are given together with pitfalls and challenges when applying machine learning approaches to the prediction of quantum-mechanical observables. [less ▲]

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See detailPair-Wise and Many-Body Dispersive Interactions Coupled to an Optimally Tuned Range-Separated Hybrid Functional
Agrawal, Piyush; Tkatchenko, Alexandre UL; Kronik, Leeor

in JOURNAL OF CHEMICAL THEORY AND COMPUTATION (2013), 9(8), 3473-3478

We propose a nonempirical, pair-wise or many-body dispersion-corrected optimally tuned range-separated hybrid functional. This functional retains the advantages of the optimal-tuning approach in the ... [more ▼]

We propose a nonempirical, pair-wise or many-body dispersion-corrected optimally tuned range-separated hybrid functional. This functional retains the advantages of the optimal-tuning approach in the prediction of the electronic structure. At the same time, it gains accuracy in the prediction of binding energies for dispersively bound systems, as demonstrated on the S22 and S66 benchmark sets of weakly bound dimers. [less ▲]

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See detailQuantification of finite-temperature effects on adsorption geometries of π-conjugated molecules: Azobenzene/Ag(111)
Mercurio, G.; Maurer, R. J.; Liu, W. et al

in Physical Review B - Condensed Matter and Materials Physics (2013), 88(3),

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The ... [more ▼]

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial. © 2013 American Physical Society. [less ▲]

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See detailOn the accuracy of van der Waals inclusive density-functional theory exchange-correlation functionals for ice at ambient and high pressures
Santra, Biswajit; Klimes, Jiri; Tkatchenko, Alexandre UL et al

in JOURNAL OF CHEMICAL PHYSICS (2013), 139(15),

Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years. However, the reliability of different DFT exchange-correlation (xc) functionals for water remains a ... [more ▼]

Density-functional theory (DFT) has been widely used to study water and ice for at least 20 years. However, the reliability of different DFT exchange-correlation (xc) functionals for water remains a matter of considerable debate. This is particularly true in light of the recent development of DFT based methods that account for van der Waals (vdW) dispersion forces. Here, we report a detailed study with several xc functionals (semi-local, hybrid, and vdW inclusive approaches) on ice Ih and six proton ordered phases of ice. Consistent with our previous study [B. Santra, J. Klimes, D. Alfe, A. Tkatchenko, B. Slater, A. Michaelides, R. Car, and M. Scheffler, Phys. Rev. Lett. 107, 185701 (2011)] which showed that vdW forces become increasingly important at high pressures, we find here that all vdW inclusive methods considered improve the relative energies and transition pressures of the high-pressure ice phases compared to those obtained with semi-local or hybrid xc functionals. However, we also find that significant discrepancies between experiment and the vdW inclusive approaches remain in the cohesive properties of the various phases, causing certain phases to be absent from the phase diagram. Therefore, room for improvement in the description of water at ambient and high pressures remains and we suggest that because of the stern test the high pressure ice phases pose they should be used in future benchmark studies of simulation methods for water. (C) 2013 AIP Publishing LLC. [less ▲]

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See detailAdsorption Geometry Determination of Single Molecules by Atomic Force Microscopy
Schuler, Bruno; Liu, Wei; Tkatchenko, Alexandre UL et al

in PHYSICAL REVIEW LETTERS (2013), 111(10),

We measured the adsorption geometry of single molecules with intramolecular resolution using noncontact atomic force microscopy with functionalized tips. The lateral adsorption position was determined ... [more ▼]

We measured the adsorption geometry of single molecules with intramolecular resolution using noncontact atomic force microscopy with functionalized tips. The lateral adsorption position was determined with atomic resolution, adsorption height differences with a precision of 3 pm, and tilts of the molecular plane within 0.2 degrees. The method was applied to five pi-conjugated molecules, including three molecules from the olympicene family, adsorbed on Cu(111). For the olympicenes, we found that the substitution of a single atom leads to strong variations of the adsorption height, as predicted by state-of-the-art density-functional theory, including van der Waals interactions with collective substrate response effects. [less ▲]

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