There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C6 coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C6 coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchangecorrelation functional. We show that the effective atomic C6 coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C6R � 6 correction method for density-functional theory calculations.

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Accurate molecular van der Waals interactions from ground-state electron density and free-atom reference data

There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

Laboratoire de Physique de la Matie `re Condense ́ , Ecole Polytechnique, 91128 Palaiseau Cedex, France LURE, CNRS-MR-CEA, Ba ˆtiment 209d, Centre Universitaire Paris-sud, Boı ˆte Postale 34, F91898, Orsay, France Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica dell’ Universita’ di Roma ‘Tor Vergata,’ Via della Ricerca Scientifica, I-00133 Roma, Italy Institut für Festkörpertheorie und Theoretische Optik, Friedrich-Schiller-Universita ̈t, Max-Wien-Platz 1, 07743 Jena, Germany ~Received 10 January 2001; revised manuscript received 22 May 2001; published 3 October 2001 !

X-ray diffraction analysis of the gallium-rich surface of GaAs(001)

The formation energies and charge transition levels of the most relevant LiNbO3 intrinsic point defects, i.e., Nb antisites and Li as well Nb vacancies, are studied from first principles. Thereby isolated defects are modeled in the framework of the density functional theory with local and hybrid exchange-correlation functionals. The inclusion of nonlocal exchange opens the LiNbO3 fundamental band gap by nearly 2 eV and modifies considerably the relative stability of the investigated defects with respect to values calculated with local functionals. On the other hand, supercell symmetry and finite-size errors in calculations using periodic boundary conditions are found to have a major influence on the outcome of the simulations. It is found that, in particular, the Nb vacancy causes a long-range strain field and requires very large supercells for its adequate modeling. Compared to previous theoretical results we find an enhanced stability of the Nb vacancy with respect to the other defects. The V−5 Nb is predicted to be stable for Fermi level positions in the upper part of the band gap.

/pdf/intrinsiclinbo3point-defects-from-hybrid-density-functional-58pwiwypal.pdf

IntrinsicLiNbO3point defects from hybrid density functional calculations

The optical anisotropy of $c(4\ifmmode\times\else\texttimes\fi{}4),$ $(2\ifmmode\times\else\texttimes\fi{}4),$ $(2\ifmmode\times\else\texttimes\fi{}6)$ and $(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructed GaAs(001) surfaces has been calculated from first principles. It consists of surface structure-dependent features originating from electronic transitions in the uppermost surface layers and of anisotropy peaks close to the ${E}_{1}$ and ${E}_{0}^{\ensuremath{'}}{/E}_{2}$ bulk critical point energies. The latter contributions are nearly structure independent and arise from transitions between surface-modified bulk electronic states. For the smaller reconstructions the influence of surface electric fields on the optical anisotropy is studied. We find that the linear electro-optic effect modifies mainly the optical anisotropy from the bulk atomic layers, resulting in changes of the reflectance anistropy spectroscopy signal which are strongly reconstruction dependent, however. Changes of the atomic relaxation due to surface electric fields are less important for the modification of the optical signal than the polarization of the electron wave functions.

/pdf/interplay-of-surface-reconstruction-and-surface-electric-43lejbhvh3.pdf

Interplay of surface reconstruction and surface electric fields in the optical anisotropy of GaAs(001)

The structure of the ( $3\ifmmode\times\else\texttimes\fi{}2$) reconstruction of $\ensuremath{\beta}$-SiC(001) surface has been identified by comparing reflectance anisotropy spectra calculated from first principles with recent measurements. Only the calculations for the two-adlayer asymmetric-dimer model agree with experiment. The two prominent peaks at 3.6 and 5.0 eV found experimentally are assigned to electronic transitions between surface and bulklike electronic states. A further pronounced anisotropy at 2.0 eV, due to transitions between surface states, is predicted.

/pdf/optical-anisotropy-of-the-sic-001-3-2-surface-evidence-for-39r50baheb.pdf

Optical Anisotropy of the SiC $001$ - $3×2$ Surface: Evidence for the Two-Adlayer Asymmetric-Dimer Model

Total-energy calculations based on density-functional theory are combined with ab initio thermodynamics to better understand the pH-value-dependent water–wollastonite(001) (CaSiO3) interaction. The truncation of wollastonite(001) is found to lead to nearly negligible ionic relaxation with respect to the bulk geometry. The thermodynamic ground state for low water coverage gives rise to a molecular adsorption energy of about 2 eV and features coexisting Si–OH and Ca–OH groups at the wollastonite(001) surface. The adsorption energy per molecule decreases to 1.4 eV with increasing the coverage to one monolayer. For water coverages in excess of one monolayer, adsorption energies close to the value characteristic for water adsorption on ice are obtained. More favorable than stoichiometric interfaces, however, are water–wollastonite(001) interfaces that are enriched in Si–OH sites: Even at basic pH values, a metal–proton exchange reaction at the water–wollastonite(001) interface lowers the total energy.

Wolf Gero Schmidt

Papers

X-ray diffraction analysis of the gallium-rich surface of GaAs(001)

IntrinsicLiNbO3point defects from hybrid density functional calculations

Interplay of surface reconstruction and surface electric fields in the optical anisotropy of GaAs(001)

Optical Anisotropy of the SiC \(001\) - \(3×2\) Surface: Evidence for the Two-Adlayer Asymmetric-Dimer Model

Formation of Hydroxyl Groups at Calcium-Silicate-Hydrate (C-S-H): Coexistence of Ca–OH and Si–OH on Wollastonite(001)