Dynamic charge distribution of pyrazine: Hartree-Fock-Slater calculation and comparison with X-ray diffraction results

The dynamic charge distribution of pyrazine (1,4-diazabenzene) has been calculated using a Hartree—Fock—Slater type wavefunction. The calculations were done in direct space employing a Taylor-series expansion. The calculated charge distribution shows approximate agreement with the results of an accurate X-ray diffraction experiment. A discussion of several factors affecting the agreement between experimental and theoretical results is given

Crystal structure and charge distribution of pyrazine: effects of extinction, thermal diffuse scattering and series termination

The crystal structure and electronic charge distribution of pyrazine (1,4-diazabenzene) has been determined at 184 K by X-ray methods. The structural results of Wheatley [Acta Cryst. (1957), 10, 182-187] have been confirmed. A clear indication of bonding effects is obtained. Neither positional and thermal parameters nor difference-Fourier maps are affected by extinction. The effect of thermal diffuse scattering (TDS) on positional parameters is also negligible. However, after correction for TDS, thermal parameters increase significantly. The difference-Fourier map is influenced by TDS as well as the inclusion of high-order Fourier terms

Molecular charge distribution of CO

The difference electron density of CO is studied by comparison of several calculations. It is shown that the Hartree-Fock-Slater and Hartree-Fock methods yield equally good charge-distributions and that the use of minimal basis sets should be avoided

The molecular structure of pyrazine as determined from gas-phase electron diffraction data

The structure of pyrazine (1,4 diazabenzene, C4H4N4) has been determined at 333 K by means of gas-phase electron diffraction. The r g parameters are as follows: r(C-C) = 1.339 ± 0.002 Å. r(C-N) = 1.403 ± 0.004 Å, r(C-H) = 1.115 ± 0.004 Å. C-C-N = 115.6 ± 0.4°, and C-C-H = 123.9 ± 0.6° (error limits are 2.5σ). At a 10% level the rα structure does not differ significantly from the structure in the solid state, so long as high order X-ray, results corrected for librational motion are used; otherwise significantly different results are found even at the 1% level. Calculated and observed mean square amplitudes compare favourably

Load-depth sensing of isotropic, linear viscoelastic materials using rigid axisymmetric indenters

An indentation experiment involves five variables: indenter shape, material behavior of the substrate, contact size, applied load and indentation depth. Only three variable are known afterwards, namely, indenter shape, plus load and depth as function of time. As the contact size is not measured and the determination of the material properties is the very aim of the test; two equations are needed to obtain a mathematically solvable system. For elastic materials, the contact size can always be eliminated once and for all in favor of the depth; a single relation between load, depth and material properties remains with the latter variable as unknown. For viscoelastic materials where hereditary integrals model the constitutive behavior, the relation between depth and contact size usually depends also on the (time-dependent) properties of the material. Solving the inverse problem, i.e., determining the material properties from the experimental data, therefore needs both equations. Extending Sneddon's analysis of the indentation problem for elastic materials to include viscoelastic materials, the two equations mentioned above are derived. To find the time dependence of the material properties the feasibility of Golden and Graham's method of decomposing hereditary integrals (J.M. Golden and G.A.C. Graham. Boundary value problems in linear viscoelasticity, Springer, 1988) is investigated and applied to a single load-unload process and to sinusoidally driven stationary state indentation processes.Comment: 116 pages, 29 figure

Note on the relation between the compressive strength of debased alumina and its use as hot-pressing die material

Durability of alumina ceramics when used as die material in compressive loading varies widely. The compressive strength of various alumina ceramics was therefore determined as a function of temperature. The results are discussed in terms of the microstructure and fracture morphology. The (hot-pressing) durability of the materials does not correlate at all with the compressive strength data obtained. Instead, this property seems to be determined entirely by the homogeneity and lack of flaws in the ceramic

Формування теоретичної моделі геополітичного дискурсу у вітчизняній політичній думці кінця ХХ – початку ХХІ століття

У статті висвітлюються питання щодо започаткування новітньої дослідницької традиції геополітичного дискурсу у проблематиці вітчизняної політичної думки ХХ – початку ХХІ століття. Зазначено позиції провідних вітчизняних вчених щодо формування емпіричного та ідейно-теоретичного підґрунтя для утвердження цієї традиції політичного дослідження.The article considers the questions of the becoming of a new research tradition of geopolitical discourse in the topic of native political thought of the 20-th – the beginning of the 21-st century. The views of leading home scientists about the development of empirical, ideological and theoretical basis for the maintenance of this tradition of political research are pointed out

Fourth order indirect integration method for black hole perturbations: even modes

On the basis of a recently proposed strategy of finite element integration in time domain for partial differential equations with a singular source term, we present a fourth order algorithm for non-rotating black hole perturbations in the Regge-Wheeler gauge. Herein, we address even perturbations induced by a particle plunging in. The forward time value at the upper node of the $(r^*,t)$ grid cell is obtained by an algebraic sum of i) the preceding node values of the same cell, ii) analytic expressions, related to the jump conditions on the wave function and its derivatives, iii) the values of the wave function at adjacent cells. In this approach, the numerical integration does not deal with the source and potential terms directly, for cells crossed by the particle world line. This scheme has also been applied to circular and eccentric orbits and it will be object of a forthcoming publication.Comment: This series of papers deals with EMRI for LISA. With the respect to the v1 version, the algorithm has been improved; convergence tests and references have been added; v2 is composed by 23 pages, and 6 figures. Paper accepted by Class. Quantum Gravity for the special issue on Theory Meets Data Analysis at Comparable and Extreme Mass Ratios (Capra and NRDA) at Perimeier Institute in June 201

Weakest-link failure prediction for ceramics IV : application of mixed-mode fracture criteria for multiaxial loading

Strength data obtained from uniaxial and biaxial bend tests on 2 alumina ceramics were analyzed by a weakest-link model with different mixed-mode fracture criteria and crack shapes. Taking the data from 3- and 4-point bend tests as a ref., strength predictions for the ball-on-ring and ring-on-ring biaxial tests were made, showing large differences in the predictions for the different fracture criteria. The best fitting models for the 2 aluminas cannot be interchanged, thus showing a marked difference in their shear stress sensitivity. Anal. shows that, in general, a combination of tests with a varying degree of stress multiaxiality is required to det. the best applicable mixed-mode fracture criterion. [on SciFinder (R)