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Yaoming Xie Henry F. Schaefer Pinchas Aped Kuohsiang Chen Norman L. Allinger 《International journal of quantum chemistry》1992,42(4):953-963
Ab initio molecular quantum mechanics has been applied to a number of coupled ring systems including biterahedryl and cubylcubane. Basis sets at least as large as double zeta augmented by carbon d functions (DZ +d) were used throughout. For biterahedryl, electron correlation effects qualitatively alter the molecular structure, decreasing the central C? C bond distance while increasing the adjacent C? C distance. Carbonyl substituents, contrary to some previous thinking, do not qualitatively alter the bitetrahedryl structure. This finding points to a structural problem with the monomer bicyclo[3.1.0.02,6]hexane that carries over to the coupled bicyclo[3.1.0.02,6]hexyl molecule. Finally, for the cubylcubane molecule synthesized in 1988, the only significant difference between Hartree–Fock theory and experiment occurs for the central C? C distance, which, as in the other coupled molecules, is too long. These results significantly lengthen the (still very short) list of closed-shell hydrocarbon molecules for which Hartree–Fock theory encounters structural difficulties. 相似文献
185.
Raissa M. D'Souza Norman H. Margolus Mark A. Smith 《Journal of statistical physics》2002,107(1-2):401-422
We introduce a simplified technique for incorporating diffusive phenomena into lattice-gas molecular dynamics models. In this method, spatial interactions take place one dimension at a time, with a separate fractional timestep devoted to each dimension, and with all dimensions treated identically. We show that the model resulting from this technique is equivalent to the macroscopic diffusion equation in the appropriate limit. This technique saves computational resources and reduces the complexity of model design, programming, debugging, simulation and analysis. For example, a reaction-diffusion simulation can be designed and tested as a one-dimensional system, and then directly extended to two or more dimensions. We illustrate the use of this approach in constructing a microscopically reversible model of diffusion-limited aggregation as well as in a model of growth of biological films. 相似文献
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An approximate analytical solution is provided for one-dimensional, counter- current, spontaneous imbibition of a wetting
phase (water) into a semi-infinite porous medium. The solution is based on the assumption that a similarity solution exists
for the displacement process. This assumption, in turn, rests on the assumption that the set of relative permeability and
capillary pressures curves are unique functions of saturation and do not depend on the nature of the displacement. It further
rests on the assumption that the saturation at the imbibition face does not vary with time. It is demonstrated that the solution
is in agreement with results obtained from experiments and also numerical analyses of these experiments. The experiments utilize
cylindrical samples with the radial surface and one end-face sealed, and with counter-current imbibition occurring at the
open end-face. The stage of the experiment that is modeled by the present solution is the period before the imbibition front
contacts the sealed end-face. An important finding of the present analysis is that the pressure upstream of the advancing
invasion front is a constant. A second, improved solution is also presented; this solution is an iterative, series solution
of an integral-differential equation. It converges to a stable solution in very few terms. 相似文献
189.
Sudhir?S.?Shenoy Norman?J.?WagnerEmail author Jonathan?W.?Bender 《Rheologica Acta》2003,42(4):287-294
A novel electrorheological (ER) effect is presented where the application of an electric field, orthogonal to the vorticity-flow plane, increases the critical hydrodynamic stress required to shear thicken concentrated, colloidal dispersions (the E-FiRST effect). The shear thickening behavior of a Brownian charge stabilized dispersion (226 nm silica in 4-methylcyclohexanol at 53, 50. and 41 vol.%) is studied in the presence of an electric field as a function of the field strength and coupling parameter ( ß) where the latter is a function of a.c. field frequency due to diffusion limitations on the polarization of the particles' double layer. A mechanism is proposed whereby the applied electric field suppresses the formation of the self-organized hydrocluster microstructure responsible for shear thickening, thus delaying the onset of shear thickening to higher applied shear stresses. A Mason-number type scaling law is found to scale the effect, which supports the proposed mechanism. 相似文献
190.
Dynamic properties of shear thickening colloidal suspensions 总被引:4,自引:0,他引:4
The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude. 相似文献