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  • American Institute of Physics (AIP)  (478)
  • American Chemical Society (ACS)
  • 1
    Electronic Resource
    Electronic Resource
    Microscopic dynamics of flow in molecularly narrow pores (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3427-3431 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458823
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  • 2
    Electronic Resource
    Electronic Resource
    Exploring potential energy surfaces with transition state calculations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4308-4319 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Means are presented for using stationary points in two ways. One, for well-understood potentials, elucidates relations between the form of the surface and the dynamics that it supports, including the determination of the effective molecular symmetry group. The other, for potentials of uncertain quality, provides a test for unphysical characteristics and suggests how the surface might be improved if it is found to be unsatisfactory in some respect. Our approach involves comparison of transition state calculations using the slowest slide and Cerjan–Miller algorithms for two example systems: the Lennard-Jones Ar7 cluster and the Handy–Carter many-body-expansion potential for the ground state of formaldehyde.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457790
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  • 3
    Electronic Resource
    Electronic Resource
    Mean and Gaussian curvatures of randomly decorated Voronoi and cubic tesselations (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3246-3251 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is now generally accepted that some midrange microemulsions are bicontinuous, i.e., continuous in both oil and water simultaneously. The first model of the microstructure of microemulsion that could account for a progression from discrete to bicontinuous was the Talmon–Prager or "randomly decorated Voronoi'' model. Space is tessellated into Voronoi polyhedra and the polyhedra are randomly decorated with oil and water. In variations of the model DeGennes and Taupin and Widom decorate a cubic tesselation of space. At first glance it might appear that the decorated Voronoi and cubic tessellations are zero-mean-curvature models, since they are constructed from polyhedra with planar faces. However, the edges of the polyhedra are concentrations of mean curvature, and the vertices are concentrations of Gaussian curvature. The area-averaged mean and Gaussian curvatures of the oil–water interface in the randomly decorated Voronoi and cubic models are calculated. The area-averaged mean curvatures of the two models are linear functions of oil volume fraction, change sign at a volume fraction of 0.5, and are within 0.2% of one another in magnitude. The area-averaged Gaussian curvature of the Voronoi model varies quadratically with volume fraction, and is negative for oil volume fractions from 0.18 to 0.82 (oil and water are bicontinuous for volume fractions ranging from 0.135 to 0.865). The area-averaged Gaussian curvature of the randomly decorated cubic model is a sixth-order polynomial function of oil volume fraction and is negative for volume fractions ranging from 0.23 to 0.77 (oil and water are bicontinuous over the volume fraction range 0.25 to 0.75). As an additional application, the model results are used to interpret curvature aspects of the bilayer theory of the L3 phase of surfactant solutions presented recently by Cates et al. [Europhys. Lett. 5, 733 (1988)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456899
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  • 4
    Electronic Resource
    Electronic Resource
    Model polarizabilities and multipoles for ionic compounds. Alkaline-earth monohalides (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1656-1663 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Törring–Ernst–Kindt (TEK) model for the alkaline-earth monohalide molecules was extended to predict molecular polarizabilities and quadrupole moments in addition to dipoles. Calculations were carried out for the 19 molecules for which experimental bond lengths and vibrational frequencies are known. It was found that for many of these molecules the TEK model predicts negative polarizability anisotropies, α⊥〉α(parallel). This is in disagreement with the dipole-induced dipole, or Silberstein, formula, but in agreement with an empirical correlation between the anisotropy and the molecular asymmetry noted by Winicur. The TEK polarizabilities give rise to a much smaller second-order Legendre anisotropy in the CaCl–Ar interaction potential than was found in earlier work using the Rittner model. The D-shell model applied earlier to the alkali halides was also applied to the 19 alkaline-earth monohalides. With a slight modification it was found to describe both classes of ionic compounds successfully, although less successfully for the alkaline-earth compounds. The D-shell model was shown to be a generalization of the TEK model which includes the effect of the shell overlap on the polarizing fields at the ions. Nevertheless, the TEK model predicts better dipole moments, unless the shell charge is treated as an additional, adjustable parameter in the D-shell model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455112
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  • 5
    Electronic Resource
    Electronic Resource
    A phase space analysis of the collinear I+HI reaction (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2429-2456 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collinear I+HI reaction is studied using an approach based on the concepts of nonlinear dynamics. Three closed regions in phase space are constructed by connecting the dynamical manifolds emanating from physically important periodic orbits. It is shown that many features of the reaction dynamics can be understood with reference to these regions. The oscillating reaction probability in this system is shown to stem from the geometrical pattern of overlap of heteroclinic oscillations of an interaction region. The process of complex formation is quantitatively described in terms of passage into a well defined complex region of phase space. The phase space representation predicts that the complex formation probability oscillates with energy and suggests that the complex lifetime might oscillate as well. We have carried out simulations which confirm both of these effects. The vibrational adiabatic approximation for the reaction is assessed relative to the exact classical dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454025
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  • 6
    Electronic Resource
    Electronic Resource
    Phase space bottlenecks: A comparison of quantum and classical intramolecular dynamics for collinear OCS (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3263-3272 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We compare quantum and classical mechanics for a collinear model of OCS at an energy (20 000 cm−1) where Davis [J. Chem. Phys. 83, 1016 (1985)] had previously found that phase space bottlenecks associated with golden mean tori inhibit classical flow between different chaotic regions in phase space. Accurate quantum eigenfunctions for this two mode system are found by diagonalizing a large basis of complex Gaussian functions, and these are then used to study the evolution of wave packets which have 20 000 cm−1 average energies. By examining phase space (Husimi) distributions associated with the wave functions, we conclude that these golden mean tori do indeed act as bottlenecks which constrain the wave packets to evolve within one (or a combination of) regions. The golden mean tori do not completely determine the boundaries between regions, however. Bottlenecks associated with resonance trapping and with separatrix formation are also involved. The analysis of the Husimi distributions also indicates that each exact eigenstate is nearly always associated with just one region, and because of this, superpositions of eigenstates that are localized within a region remain localized in that region at all times. This last result differs from the classical picture at this energy where flow across the bottlenecks occurs with a 2–4 ps lifetime. Since the classical phase space area through which flux must pass to cross the bottlenecks is small compared to (h-dash-bar) for OCS, the observed difference between quantum and classical dynamics is not surprising. Examination of the time development of normal mode energies indicates little or no energy flow quantum mechanically for wave packet initial conditions. Classical trajectory bundles constructed from the wave packet phase space distributions also show little or no energy flow even though noticeable flow is observed for more localized bundles chosen from the turnstile associated with flow through the bottleneck.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451985
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  • 7
    Electronic Resource
    Electronic Resource
    Melting and freezing in isothermal Ar13 clusters (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6456-6464 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microcanonical simulations have shown that Ar13 clusters have sharp but unequal melting and freezing energies. Between these energies, a hot solid-like form and a cooler, liquid-like form coexist in dynamic equilibrium. Monte Carlo and isothermal molecular dynamics simulations confirm that this coexistence behavior persists under canonical conditions as well. Many properties demonstrate the solid and liquid character of the two coexisting "phases.'' One previous result seemed to contradict this: Quirke and Sheng evaluated nearest neighbor angular distribution function P(θ); its nonzero value for θ=π/2 at 33 K was interpreted as that of a hot solid in a "premelting expansion.'' Actually, that result is the average of a bimodal distribution, one mode for the solid and the other for the liquid. The average shifts smoothly with T, and each form's P(θ) changes slightly with temperature. The solid has tiny nonzero probability for π/2. The liquid has a minimum probability there, but far above zero. Mean-square displacements and power spectra calculated at 33 K from the Nosé constant temperature molecular dynamics method exhibit properties which are clearly distinguishable and identifiable with two distinct phases, as they are under isoergic conditions. Hence our results can be added to the evidence supporting the picture for finite systems of two phases coexisting over a finite temperature and energy range.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452436
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  • 8
    Electronic Resource
    Electronic Resource
    Model polarizabilities and multipoles for ionic compounds. Alkali halides (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 1080-1087 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Various dipole interaction models were compared with one another and with literature values for dissociation energies, multipole moments (u, Q, and Ω), and polarizabilities α for the alkali halide molecules. A displaced-shell (D-shell) model, which takes account of the overlap repulsion as in the shell model but also uses the effective electron shell displacements in calculating the electrostatic interactions, gives slightly better dissociation energies and as good or better dipole and quadrupole moments than the other models. All of the models considered here give mean polarizabilities in fairly good agreement with published SCF values; however, none gives consistently good polarizability anisotropies. It is found that both the overlap repulsion (as treated in the shell model) and the effective displacements of the electron shells (as treated in the D-shell model) have large effects on the calculated polarizability anisotropies relative to the DID model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454226
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular dynamics of flow in micropores (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 1733-1750 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in a micropore only a few molecular diameters wide. Fluid inhomogeneity is introduced by imposing an external potential that in one case simulates flat solid walls and in the other case causes density peaks in the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 1010–1011 s−1 of the simulation. The Green–Kubo relation is found to be valid for the diffusivity under the flow studied. We propose a local average density model (LADM) of viscosity and diffusivity, in which the local transport coefficients are those of homogeneous fluid at a mean density obtained by averaging the local density over a molecular volume. LADM predicts qualitatively correct velocity profiles, effective viscosities, and shear stresses using only equilibrium density profiles and molecular diameters. An analogous local equilibrium version of Enskog's theory of diffusivity agrees well with the simulated pore diffusivities. Recently Vanderlick and Davis generalized Enskog's theory of diffusivity to strongly inhomogeneous fluids. Their theoretical pore diffusion coefficient is also in good agreement with simulation results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453240
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  • 10
    Electronic Resource
    Electronic Resource
    Gradient theory of the electric double layer at hydrocarbon–water interfaces (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6120-6127 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Gradient theory of inhomogeneous fluids is used to model the electric double layer at a hydrocarbon–aqueous electrolyte interface. It is applied to a four-component regular solution (two solvent components and two ionic species) in which the dielectric permittivity is also inhomogeneous. The permittivity is determined by a Clausius–Mossotti constitutive equation based on the local densities of the solvent species. The gradient equations are coupled to Poisson's equation and are solved using a Galerkin/finite element scheme. The results indicate that in dilute solution, the dependence of the interfacial tension on bulk electrolyte concentration is linear, and the sign and magnitude of the deviation of the tension from the pure solvent value depends strongly on the degree of ion partitioning between the two phases. In more concentrated solutions, the deviation becomes nonlinear and can change sign as the electrolyte concentration is increased.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453486
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