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Angyan J.G.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling
Current Organic Chemistry

The concept of localization and delocalization in molecules is discussed in terms of the response of the electronic system to an external perturbation. It is argued that both the spatial organization of electrons in pairs and the spatial distribution of the response intensity, reflect main features of the correlated motion of electrons, ultimately described by the pair distribution function of electrons. Various measures, derived from the linear charge density response function, are able to characterize localization in a rigorous way, in close analogy to the approach followed in solid state physics. © 2011 Bentham Science Publishers. Source

Lebegue S.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling | Bjorkman T.,Aalto University | Klintenberg M.,Uppsala University | Nieminen R.M.,Aalto University | Eriksson O.,Uppsala University
Physical Review X

Progress in materials science depends on the ability to discover new materials and to obtain and understand their properties. This has recently become particularly apparent for compounds with reduced dimensionality, which often display unexpected physical and chemical properties, making them very attractive for applications in electronics, graphene being so far the most noteworthy example. Here, we report some previously unknown two-dimensional materials and their electronic structure by data mining among crystal structures listed in the International Crystallographic Structural Database, combined with density-functional-theory calculations. As a result, we propose to explore the synthesis of a large group of two-dimensional materials, with properties suggestive of applications in nanoscale devices, and anticipate further studies of electronic and magnetic phenomena in low-dimensional systems. Source

Ping Y.,University of California at Davis | Rocca D.,University of California at Davis | Rocca D.,University of Lorraine | Rocca D.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling | Galli G.,University of California at Davis
Chemical Society Reviews

We describe state of the art methods for the calculation of electronic excitations in solids and molecules, based on many body perturbation theory, and we discuss some applications of these methods to the study of band edges and absorption processes in representative materials used as photoelectrodes for water splitting. © 2013 The Royal Society of Chemistry. Source

Mata I.,CSIC - Institute of Materials Science | Alkorta I.,Institute Quimica Medica IQM CSIC | Molins E.,CSIC - Institute of Materials Science | Espinosa E.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling
Chemistry - A European Journal

Topological analyses of the theoretically calculated electron densities for a large set of 163 hydrogenbonded complexes show that H··· X interactions can be classified in families according to X (X=atom or π orbital). Each family is characterised by a set of intrinsic dependencies between the topological and energetic properties of the electron density at the hydrogenbond critical point, as well as between each of them and the bonding distance. Comparing different atom-acceptor families, these dependencies are classified as a function of the van der Waals radius rX or the electronegativity χX, which can be explained in terms of the molecular orbitals involved in the interaction. According to this ordering, the increase of χX leads to a larger range of H···X distances for which the interaction is of pure closed-shell type. Same dependencies observed for H···O interactions experimentally characterised by means of high-resolution X-ray diffraction data show a good agreement with those obtained from theoretical calculations, in spite of a larger dispersion of values around the expected fitting functions in the experimental case. Theoretical dependencies can thus be applied to the analysis of the experimental electron density for detecting either unconventional hydrogen bonds or problems in the modelling of the experimental electron density. © 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim. Source

Bucko T.,Comenius University | Bucko T.,Slovak Academy of Sciences | Lebegue S.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling | Hafner J.,University of Vienna | Angyan J.G.,CNRS Laboratory of Crystallography, Nuclear Magnetic Resonance and Modelling
Physical Review B - Condensed Matter and Materials Physics

The method proposed by Tkatchenko and Scheffler to correct density functional calculations for the missing van der Waals interactions is implemented in the Vienna ab initio simulation package (vasp) code and tested on a wide range of solids, including noble-gas crystals, molecular crystals (α-N2, sulfur dioxide, benzene, naphthalene, cytosine), layered solids (graphite, hexagonal boron nitride, vanadium pentoxide, MoS2, NbSe2), chain-like structures (selenium, tellurium, cellulose I), ionic crystals (NaCl, KI), and metals (nickel, zinc, cadmium). In addition to the original formulation expressing the van der Waals (vdW) corrections as pairwise potentials whose strength is derived from the rescaled polarizabilities of the neutral free atoms, the self-consistently screened (TS+SCS) variant of the method involving electrodynamic response effects has been examined. Analytical expressions for the forces acting on the atoms and for the components of the stress tensor needed for the relaxation of the volume and shape of the unit cell using the TS+SCS method are derived. While the calculated structures are reasonably close to experiment, the van der Waals corrections to the binding energies are often found to be overestimated in comparison with experimental data. The TS+SCS approach leads to significantly better results in some problematic cases, such as the binding energy of graphite. However, there is room for further improvements, in particular for strongly ionic systems. © 2013 American Physical Society. Source

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