Kinetic Energy Density Study of Some Representative Semilocal Kinetic Energy Functionals

There is a number of explicit kinetic energy density functionals for non-interacting electron systems that are obtained in terms of the electron density and its derivatives. These semilocal functionals have been widely used in the literature. In this work we present a comparative study of the kinetic energy density of these semilocal functionals, stressing the importance of the local behavior to assess the quality of the functionals. We propose a quality factor that measures the local differences between the usual orbital-based kinetic energy density distributions and the approximated ones, allowing to ensure if the good results obtained for the total kinetic energies with these semilocal functionals are due to their correct local performance or to error cancellations. We have also included contributions coming from the laplacian of the electron density to work with an infinite set of kinetic energy densities. For all the functionals but one we have found that their success in the evaluation of the total kinetic energy are due to global error cancellations, whereas the local behavior of their kinetic energy density becomes worse than that corresponding to the Thomas-Fermi functional.Comment: 12 pages, 3 figure

Electronic band structure, Fermi surface, and elastic properties of new 4.2K superconductor SrPtAs from first-principles calculations

The hexagonal phase SrPtAs (s.g. P6/mmm; #194) with a honeycomb lattice structure very recently was declared as a new low-temperature (TC ~ 4.2K) superconductor. Here by means of first-principles calculations the optimized structural parameters, electronic bands, Fermi surface, total and partial densities of states, inter-atomic bonding picture, independent elastic constants, bulk and shear moduli for SrPtAs were obtained for the first time and analyzed in comparison with the related layered superconductor SrPt2As2.Comment: 8 pages, 4 figure

{\it Ab initio} 27Al^{27}Al NMR chemical shifts and quadrupolar parameters for Al2O3Al_2O_3 phases and their precursors

The Gauge-Including Projector Augmented Wave (GIPAW) method, within the Density Functional Theory (DFT) Generalized Gradient Approximation (GGA) framework, is applied to compute solid state NMR parameters for $^{27}Al$ in the $\alpha$, $\theta$, and $\kappa$ aluminium oxide phases and their gibbsite and boehmite precursors. The results for well-established crystalline phases compare very well with available experimental data and provide confidence in the accuracy of the method. For $\gamma$-alumina, four structural models proposed in the literature are discussed in terms of their ability to reproduce the experimental spectra also reported in the literature. Among the considered models, the $Fd\bar{3}m$ structure proposed by Paglia {\it et al.} [Phys. Rev. B {\bf 71}, 224115 (2005)] shows the best agreement. We attempt to link the theoretical NMR parameters to the local geometry. Chemical shifts depend on coordination number but no further correlation is found with geometrical parameters. Instead our calculations reveal that, within a given coordination number, a linear correlation exists between chemical shifts and Born effective charges

Monitoring the Dynamics of Scaled Vehicles Using a Sonic Digitizer

Certain dynamic stability characteristics of an alternate method of mounting a front-end loader to a farm tractor were compared to those of a conventionally mounted front-end loader operating on a specified terrain. One objective of the study was to determine if the alternate method of mounting a front-end loader resulted in better stability characteristics and, in turn, safer operation than a conventional front-end loader. Another objective was to determine if a three-dimensional sonic digitizer could monitor the motion of the scale model tractor-loader systems accurately enough to perform a comparison between the systems. This article describes the second objective of the study. A battery-powered, 1/4 scale model tractor-loader was used to perform experimental tests. Both tractor-loader systems were operated on two slopes and on random combinations of two sinusoidal bump heights, two load weights, two loader heights, and two velocities. Three replications were performed of each test condition. A three-dimensional sonic digitizer was utilized to monitor the motion of the scale model tractor-loader systems. The sonic digitizing system made it possible to record position of the tractor-loader systems versus time at levels of frequency and accuracy faster than any previous methods of obtaining this type of data. This greater number of observations allowed systems to be statistically compared, which was not possible with previous data collection systems. The digitizing system was able to locate each sound emitter accurately. Elapsed time between emitter firing sequences may have resulted in the measured roll and front axle rotation angles to be less than actual peaks

Dynamic Stability of Two Tractor Front-End Loader Systems

A one-quarter scale model tractor and loader were designed, fabricated, and used in an experiment to evaluate two tractor-loader configurations relative to stability using roll angle as the means of assessment. The nonconventional loader system consisted of a conventional loader attached to a steerable carrier which in turn was attached to the tractor by the front axle and drawbar of the tractor. The stabilizing axle for the non-conventional loader system was the front axle as opposed to the conventional system in which the stabilizing axle was the rear axle. The experiment showed that the non-conventional loader had an inherent advantage in stability as assessed by roll angle. The roll angle was smaller at higher velocity than at lower velocity because the tires underwent greater deformation at the higher velocity than at the lower velocity

Understanding the Clean Interface between Covalent Si and Ionic Al2O3

The atomic and electronic structures of the (001)-Si/(001)-gamma-Al2O3 heterointerface are investigated by first principles total energy calculations combined with a newly developed "modified basin-hopping" method. It is found that all interface Si atoms are fourfold coordinated due to the formation of Si-O and unexpected covalent Si-Al bonds in the new abrupt interface model. And the interface has perfect electronic properties in that the unpassivated interface has a large LDA band gap and no gap levels. These results show that it is possible to have clean semiconductor-oxide interfaces

Further Characterization of Dopamine Release by Permeabilized PC 12 Cells

Rat pheochromocytoma cells (PC 12) permeabilized with staphylococcal α-toxin release [3H]dopamine after addition of micromolar Ca2+. This does not require additional Mg2+-ATP (in contrast to bovine adrenal medullary chromaffin cells). We also observed Ca2+-dependent [3H]-dopamine release from digitonin-permeabilized PC 12 cells. Permeabilization with α-toxin or digitonin and stimulation of the cells were done consecutively to wash out endogenous Mg2+-ATP. During permeabilization, ATP was removed effectively from the cytoplasm by both agents but the cells released [3H]dopamine in response to micromolar Ca2+ alone. Replacement by chloride of glutamate, which could sustain mitochondrial ATP production in permeabilized cells, does not significantly alter catecholamine release induced by Ca2+. However, Mg2+ without ATP augments the Ca2+-induced release. The release was unaltered by thiol-, hydroxyl-, or calmodulin-interfering substances. Thus Mg2+-ATP, calmodulin, or proteins containing -SH or -OH groups are not necessary for exocytosis in permeabilized PC 12 cells

Comparison Between Hydrogen and Halogen Bonds in Complexes of 6-OX-Fulvene with Pnicogen and Chalcogen Electron Donors

Quantum chemical calculations are applied to complexes of 6‐OX‐fulvene (X=H, Cl, Br, I) with ZH3/H2Y (Z=N, P, As, Sb; Y=O, S, Se, Te) to study the competition between the hydrogen bond and the halogen bond. The H‐bond weakens as the base atom grows in size and the associated negative electrostatic potential on the Lewis base atom diminishes. The pattern for the halogen bonds is more complicated. In most cases, the halogen bond is stronger for the heavier halogen atom, and pnicogen electron donors are more strongly bound than chalcogen. Halogen bonds to chalcogen atoms strengthen in the order

Fully-dynamic Approximation of Betweenness Centrality

Betweenness is a well-known centrality measure that ranks the nodes of a network according to their participation in shortest paths. Since an exact computation is prohibitive in large networks, several approximation algorithms have been proposed. Besides that, recent years have seen the publication of dynamic algorithms for efficient recomputation of betweenness in evolving networks. In previous work we proposed the first semi-dynamic algorithms that recompute an approximation of betweenness in connected graphs after batches of edge insertions. In this paper we propose the first fully-dynamic approximation algorithms (for weighted and unweighted undirected graphs that need not to be connected) with a provable guarantee on the maximum approximation error. The transfer to fully-dynamic and disconnected graphs implies additional algorithmic problems that could be of independent interest. In particular, we propose a new upper bound on the vertex diameter for weighted undirected graphs. For both weighted and unweighted graphs, we also propose the first fully-dynamic algorithms that keep track of such upper bound. In addition, we extend our former algorithm for semi-dynamic BFS to batches of both edge insertions and deletions. Using approximation, our algorithms are the first to make in-memory computation of betweenness in fully-dynamic networks with millions of edges feasible. Our experiments show that they can achieve substantial speedups compared to recomputation, up to several orders of magnitude

Molecules in clusters: the case of planar LiBeBCNOF built from a triangular form LiOB and a linear four-center species FBeCN

Krueger some years ago proposed a cluster LiBeBCNOF, now called periodane. His ground-state isomer proposal has recently been refined by Bera et al. using DFT. Here, we take the approach of molecules in such a cluster as starting point. We first study therefore the triangular molecule LiOB by coupled cluster theory (CCSD) and thereby specify accurately its equilibrium geometry in free space. The second fragment we consider is FBeCN, but treated now by restricted Hartree-Fock (RHF) theory. This four-center species is found to be linear, and the bond lengths are obtained from both RHF and CCSD calculations. Finally, we bring these two entities together and find that while LiOB remains largely intact, FBeCN becomes bent by the interaction with LiOB. Hartree-Fock and CCSD theories then predict precisely the same lowest isomer found by Bera et al. solely on the basis of DFT.Comment: to appear in Phys. Lett.