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  • American Institute of Physics (AIP)  (118)
  • Nature Publishing Group  (59)
  • 1985-1989  (177)
  • 1
    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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  • 2
    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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Molecular dynamics for discontinuous potential. IV. Lennard-Jonesium (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4307-4313 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Argon, represented as a Lennard-Jones fluid, is simulated using a discontinuous version of the potential. The effect of discretization is thoroughly investigated in terms of the number of discontinuities and the cutoff diameter. Radial distribution functions, pressure, energy, self-diffusion coefficients, and velocity autocorrelation functions are used in order to evaluate this effect. A reduction of five times on computing time can be achieved by varying the cutoff radius and the number of discontinuities without much departure from the physical representation of Lennard-Jonesium atoms. The results indicate that discontinuous molecular dynamics is a viable alternative method to Monte Carlo and continuous molecular dynamics in simulating the properties of simple fluids.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456811
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  • 5
    Electronic Resource
    Electronic Resource
    The statistical mechanics of inhomogeneous hard rod mixtures (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7136-7145 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The exact statistical mechanical solution to the problem of an equilibrium inhomogeneous classical one-dimensional mixture of hard rods is presented. From the solution, thermodynamic properties, density profiles, and correlation functions of hard rod fluids confined to small regions (micropores) can be calculated. The theory is applied to investigate microstructure, pore pressures, and pore adsorption selectivity of micropores in equilibrium with binary hard rod mixtures. A prescription is suggested for generalizing the one-dimensional results to higher dimensions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457329
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  • 6
    Electronic Resource
    Electronic Resource
    Molecular theories of confined fluids (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2422-2436 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density profiles and normal pressures predicted by three different approximate density functional free energy theories and the Fischer–Methfessel approximation to the Yvon–Born–Green (YBG) equation are compared with computer simulation results for fluids confined between planar walls. All models require as input a homogeneous fluid equation of state. Comparisons are made using two mean-field equations of state, one based on a Clausius hard-sphere reference fluid and the other based on a Carnahan/Starling hard-sphere reference fluid. The simplest and oldest of the models, the generalized van der Waals model, becomes unphysical at high mean pore densities. The Carnahan/Starling version of Tarazona model agrees best overall with the simulations. This model represents a systematic improvement on the generalized van der Waals model and is computationally the most complicated of all models examined. The YBG and generalized hard-rod models are not as accurate as the Tarazona model, but they capture the qualitative trends observed in the simulations. Both of these models are intuitive extensions of the exact theory of one-dimensional hard rods, and are computationally much simpler than the Tarazona model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455985
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  • 7
    Electronic Resource
    Electronic Resource
    Efficient molecular simulation of chemical potentials (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2370-2385 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper evaluates methods of estimating chemical potential—actually fugacity coefficient—from molecular simulations. These methods are based on formulas given by Widom, by Bennett, and by Shing and Gubbins. They are tested with molecular dynamics simulations of Lennard-Jones liquids along the isotherm kT/ε=1.2 for densities from 0.65σ−3 to 0.90σ−3. A new test molecule sampling method, excluded volume map sampling, is found to be as much as 2 orders of magnitude more efficient than uniform test molecule sampling; it is more efficient and reliable than restricted umbrella sampling proposed by Shing and Gubbins. Several difficulties of estimating the variance of the fugacity coefficient estimates are surmounted by novel application of standard statistical methods. The statistical analysis shows that Bennett's method yields estimates with the least variance and Widom's method yields estimates with the greatest variance although all methods are consistent to within statistical error. Pressure and fugacity coefficient estimates obey the Gibbs–Duhem equation along the chosen isotherm up to densities of 0.75σ−3 for 108-molecule, 0.8σ−3 for 200-molecule, and 0.9σ−3 for 500-molecule simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455979
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  • 8
    Electronic Resource
    Electronic Resource
    Coriolis induced vibration and rotation mixing in formaldehyde (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 5420-5433 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of Coriolis interaction on the intramolecular dynamics of formaldehyde is investigated theoretically. Classical dynamics calculations are presented that include all seven degrees of freedom, i.e., the six vibrational modes and the rotational mode associated with K, the projection of total angular momentum on a body-fixed axis. These calculations show that vibrational modes 4 (out of plane bend) and 6 (HCO bend) can be significantly coupled due to Coriolis interaction. The qualitative features of such coupling is further understood via study of a reduced dimension model that includes just modes 4, 6, and the rotational degree of freedom. Two interesting classical consequences of Coriolis interaction are noted. First, for large K (e.g., for ||K||∼J) we find vibrational mixing can occur without significant deterioration of K. We find that this mixing is not due to the usual "Chirikov'' or pendulum picture of nonlinear resonance. Second, we find that both vibrational and rotational mixing can occur when the rotational frequency is in resonance with the difference in vibrational frequencies. In this situation, the resonances lead to a partial K mixing. Chaos about the resonances can lead to a more extensive K mixing. Quantum calculations are also performed for the reduced dimension model and the nature of the quantum eigenstates is discussed in relation to the corresponding classical dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456448
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  • 9
    Electronic Resource
    Electronic Resource
    Measurement of the 2D5/2–2D3/2 fine structure interval in metastable nitrogen atoms at 1.15 mm by laser magnetic resonance (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 628-632 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Far infrared laser magnetic resonance (LMR) spectra of metastable (2p3)2D nitrogen atoms have been measured using laser lines in carbonyl fluoride (COF2) at 1135 μm and methanol (CH3OH) at 1224 μm. The concentration of N 2D was optimized by electron paramagnetic resonance (EPR) using a hybrid EPR/LMR spectrometer. The far infrared cavity of this spectrometer was designed to oscillate at wavelengths longer than 1 mm. The LMR spectra were analyzed using g factors derived from the EPR spectrum [Radford and Evenson, Phys. Rev. 168, 70 (1968)] and yield a precise value of the 2D5/2–2D3/2 fine structure separation of −261 447.7(2) MHz, −8.720 957(7) cm−1(2σ).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456141
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  • 10
    Electronic Resource
    Electronic Resource
    A tractable molecular theory of flow in strongly inhomogeneous fluids (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3152-3162 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently introduced model is used to study several flows in fluids with large density variations over distances comparable to their molecular dimensions (strongly inhomogeneous fluids). According to our model, the local average density model (LADM), local viscosity coefficients can be assigned at each point in a strongly inhomogeneous fluid and the stress tensor retains its Newtonian form provided that the properly defined local viscosities are used. The model has been previously shown to agree with the results of molecular dynamics simulations on diffusion and flow properties in plane Couette flow. Application of this model requires determination of the molecular density profiles in the flow region. Using a successful closure for the pair distribution function, we solve the Yvon–Born–Green (YBG) equation of fluid structure in order to determine the density profiles of a fluid confined between planar micropore walls only a few molecular diameters apart. The fluid confinement produces a strongly inhomogeneous structure. Subsequently we apply LADM to set up the fluid mechanical equations for Couette flow, Poiseuille flow, and squeezing flow between parallel plates. With the use of the YBG theoretical density profiles we solve the flow equations and predict velocity profiles, stress distributions, and effective viscosities. The dependence of these quantities on the fluid inhomogeneity is described. The effective viscosity of strongly inhomogeneous fluids is found to be quite sensitive to the nature of the flow. Our squeezing flow analysis provides a first explanation of recent experimental findings on the effective viscosity of simple fluids confined in very narrow spaces.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454972
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