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91.
Self-consistent Hartree-Fock and generalized valence bond calculations have been performed on clusters modeling the (111) silicon surface. We find that the surface state is accurately described as a dangling bond surface orbital with 93% p character. We determined the optimum relaxation of the surface layer to be 0.08 Å toward the second layer. In the positive ion, the surface atom relaxes toward the second layer by an additional 0.30 Å, and for the negative ion the surface atom moves toward the vacuum 0.25 Å. The vertical ionization potential was found to be 5.78 eV (experimental values are 5.6 – 5.9 eV) while the calculated adiabatic electron affinity is 3.02 eV. 相似文献
92.
93.
R.P. Goddard 《Nuclear Physics A》1977,291(1):13-20
Deuteron-nucleus optical model calculations which include two types of tensor spin-orbit interactions are compared. The interactions considered are of the forms where the operators Tr and Tp are defined as . Here, are the deuteron spin, position, and momentum operators, respectively. If a physically reasonable radial dependence VTP(r) is used, the S-matrix elements (and hence all observables) calculated using a Tp tensor potential are found to be similar to those calculated using a suitably chosen Tr potential, provided that the incident deuteron energy is small compared to the real central potential. At higher deuteron energies, the differences between the effects of Tr and Tp potentials are more pronounced. 相似文献
94.
Extending recent work of Brink and Olive, an off-mass shell physical state projection operator for the critical dimension, d = 26, of space-time, for the conventional dual resonance model is constructed. It can be used to give a direct calculation of the single planar dual loop amplitude, and to express amplitudes entirely in terms of physical state creation operators. 相似文献
95.
Summary Stiffness is an important subjective phenomenon diagnostically and therapeutically in rheumatic diseases such as rheumatoid arthritis and osteoarthrosis. A quantitative and qualitative assessment of these phenomena have been made by arthrographs which impose a sinusoidal motion on the metacarpo-phalangeal and knee joints respectively and record the torque in relation to the displacement. The reproducibility with the joint in situ and moved is satisfactory. A hysteresis torque/ displacement plot is obtained with both joints. Reflex muscle activity may be measured at the joint, as in myotonia congenita and Parkinsonism. In patients without neuromuscular disorders electromyographic studies have shown that the muscles are electrically silent during sinusoidal motion and that active contraction distorts the tracing. The latter fact may be useful in the evaluation of patients with psychological factors producing muscle tension, which may be decreased with diazepam. Using the cat's wrist joint it has been shown that the passive stretching of muscles accounts for 41 per cent of the stiffness recorded at the joint and the capsule 47 per cent, a finding in keeping with work on the distal interphalangeal joint of the human finger.Stiffness increases with advancing age and decreased temperature. Conversely, warming the joint reduces its stiffness. Articular gelling in patients with osteoarthrosis may be demonstrated after periods of rest and the results suggest that such changes are due to alterations in the periarticular and muscle tissues. The rapid diminution in torque after rest in one position was not detected in normal joints even after longer periods of immobilisation.
Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970. 相似文献
Zusammenfassung Steifheit ist ein Problem, das dem Kliniker bekannt ist. Steifheit am Morgen ist ein Hauptgesichtspunkt für die Diagnose von rheumatischer Arthritis und ist ein äußerst wichtiges Merkmal für die Diagnose von Wirbelsäulenversteifungs (pocker back). Diese ist ein wichtiges Merkmal in Fällen von Patienten mit Osteoarthrosis, die unter Gelenkschwellung leiden, und die Gelenke daher nach einer Ruheperiode viel steifer sind. Therapeutisch hat sie Bedeutung für das Symptomenbild des Patienten und sie ist ein Maßstab für die Wirksamkeit der Behandlung von rheumatischer Arthritis.Arthrographen wurden angefertigt, um die Steifheit des Metacarpophalangealgelenks des Fingers und des Kniegelenkes zu messen. Tierexperimente haben gezeigt, daß 50% der Gelenksteifheit von passiver Muskelstrek-kung und 50% von Geweben im Gelenk selbst und außerhalb herrühren. Reflexbedingte Muskelaktivität spielt keine Rolle, mit Ausnahme von Fällen mit neuro-muskulären Erkrankungen wieParkinsonismus und Myotonia Congenita. Um ein charakteristisches Spannungs-Dehnungsdiagramm zu erhalten, wird das Gelenk in eine sinusartige Bewegung gebracht. Untersuchungen der Spannungsrelaxation haben einige Hinweise auf die Komponenten der physischen Steifheit gegeben.Zunehmende Steifheit ist physiologisch altersbedingt. Eine Temperaturerhöhung auf 45 °C vermindert die Steifheit um etwa 25%, und eine Temperaturerniedrigung erhöht, im Gegensatz dazu, die Steifheit um etwa denselben Betrag. Erhöhte Steifheit ist pathologisch in Fällen mit rheumatischer Arthritis, Systemsklerose und Osteoarthrose. Das Auftreten von Gelenkschwellungen bei Osteoarthrose ist, wie die Analyse von Überlagerungsdiagrammen andeutet, mehr eine Erscheinung, die von Veränderungen der umhüllenden Gewebe herrührt, als von Veränderungen zwischen den Gelenkflächen. Mit diesem Gerät wurden einige künstliche Schmiermittel geprüft.
Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970. 相似文献
96.
97.
Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field 总被引:1,自引:0,他引:1
Chenoweth K Cheung S van Duin AC Goddard WA Kober EM 《Journal of the American Chemical Society》2005,127(19):7192-7202
To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH(3) radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH(3) radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(3) radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO(2) slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C(2) hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers. 相似文献
98.
Balaban TS Linke-Schaetzel M Bhise AD Vanthuyne N Roussel C Anson CE Buth G Eichhöfer A Foster K Garab G Gliemann H Goddard R Javorfi T Powell AK Rösner H Schimmel T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(8):2267-2275
We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes. 相似文献
99.
100.