31,873 research outputs found
A square-well model for the structural and thermodynamic properties of simple colloidal systems
A model for the radial distribution function of a square-well fluid of
variable width previously proposed [S. B. Yuste and A. Santos, J. Chem. Phys.
{\bf 101}, 2355 (1994)] is revisited and simplified. The model provides an
explicit expression for the Laplace transform of , the coefficients
being given as explicit functions of the density, the temperature, and the
interaction range. In the limits corresponding to hard spheres and sticky hard
spheres the model reduces to the analytical solutions of the Percus-Yevick
equation for those potentials. The results can be useful to describe in a fully
analytical way the structural and thermodynamic behavior of colloidal
suspensions modeled as hard-core particles with a short-range attraction.
Comparison with computer simulation data shows a general good agreement, even
for relatively wide wells.Comment: 23 pages, 10 figures; Figs. 4 and 5 changed, Fig. 6 new; to be
published in J. Chem. Phy
Biological Individuals
The impressive variation amongst biological individuals generates many complexities in addressing the simple-sounding question what is a biological individual? A distinction between evolutionary and physiological individuals is useful in thinking about biological individuals, as is attention to the kinds of groups, such as superorganisms and species, that have sometimes been thought of as biological individuals. More fully understanding the conceptual space that biological individuals occupy also involves considering a range of other concepts, such as life, reproduction, and agency. There has been a focus in some recent discussions by both philosophers and biologists on how evolutionary individuals are created and regulated, as well as continuing work on the evolution of individuality
Molecular simulation of the phase behavior of noble gases using accurate two-body and three-body intermolecular potentials
Gibbs ensemble Monte Carlo simulations are reported for the vapor- liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole-dipole-quadrupole, dipole- quadrupole-quadrupole, quadrupole-quadrupole-quadrupole, and fourth- order triple- dipole terms. It is shown that vapor-liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid- phase densities are predicted accurately. (C) 1999 American Institute of Physics. S0021- 9606(99)50728-9
Heat capacity of square-well fluids of variable width
We have obtained by Monte Carlo NVT simulations the constant-volume excess
heat capacity of square-well fluids for several temperatures, densities and
potential widths. Heat capacity is a thermodynamic property much more sensitive
to the accuracy of a theory than other thermodynamic quantities, such as the
compressibility factor. This is illustrated by comparing the reported
simulation data for the heat capacity with the theoretical predictions given by
the Barker-Henderson perturbation theory as well as with those given by a
non-perturbative theoretical model based on Baxter's solution of the
Percus-Yevick integral equation for sticky hard spheres. Both theories give
accurate predictions for the equation of state. By contrast, it is found that
the Barker-Henderson theory strongly underestimates the excess heat capacity
for low to moderate temperatures, whereas a much better agreement between
theory and simulation is achieved with the non-perturbative theoretical model,
particularly for small well widths, although the accuracy of the latter worsens
for high densities and low temperatures, as the well width increases.Comment: 11 pages, 4 figures; figures now include additional perturbation
data; to be published in Mol. Phy
Theoretical three-and four-axis gimbal robot wrists
In high-performance flight simulations, a four-axis gimbal system allows all possible rotations with acceptable gimbal angle rates while it avoids the so-callled 'gimbal lock' that occurs when gimbal rotational axes are colinear. In this paper, pertinent equations (including quaternions) are assembled for a hypothetical robot wrist, functionally equivalent to this four-axis gimbal system, and also for a true three-axis gimbal robot wrist. These equations are used to simulate the rotation of a robot hand by the robot wrist in response to operator rotational velocity commands to the robot hand. Near gimbal lock (wrist singularity), excessive rotational rates occur. Scaling the rates, which is necessary for the three-gimbal robot wrist to prevent rate limiting, introduces an undesirable time delay in the robot hand rotation with respect to the commanded rotation. However, the merit of the four-gimbal robot wrist is that the fourth gimbal angle keeps the robot wrist away from the singularity so that the robot hand moves exactly as commanded. It appears that in a 'worst-type' maneuver of the robot hand, the fourth gimbal angle can be defined so that none of the gimbal angle rates exceed about twice the commanded rates
R-matrix and K-matrix analysis of elastic alpha-alpha scattering
The R- and K-matrix parametrizations are analyzed and compared for the
elastic alpha-alpha scattering at center-of-mass energies below 40 MeV. The two
parametrizations differ in their definitions of the resonance energy which can
lead to quite different results. The physical values of the best-fit parameters
are compared with those computed for a potential model. The existence of a
broad resonance near 9 MeV is not supported by the data or by the potential
model. We discuss the positive and negative aspects for both parametrizations.Comment: 14 pages with 4 figure
Exploiting correlogram structure for robust speech recognition with multiple speech sources
This paper addresses the problem of separating and recognising speech in a monaural acoustic mixture with the presence of competing speech sources. The proposed system treats sound source separation and speech recognition as
tightly coupled processes. In the first stage sound source separation is performed in the correlogram domain. For periodic sounds, the correlogram exhibits symmetric tree-like structures whose stems are located on the delay
that corresponds to multiple pitch periods. These pitch-related structures are exploited in the study to group spectral components at each time frame. Local
pitch estimates are then computed for each spectral group and are used to form simultaneous pitch tracks for temporal integration. These processes segregate a spectral representation of the acoustic mixture into several time-frequency regions such that the energy in each region is likely to have originated from a single periodic sound source. The identified time-frequency regions, together
with the spectral representation, are employed by a `speech fragment decoder' which employs `missing data' techniques with clean speech models to simultaneously search for the acoustic evidence that best matches model sequences. The paper presents evaluations based on artificially mixed simultaneous speech utterances. A coherence-measuring experiment is first reported which quantifies the consistency of the identified fragments with a single source. The system is then evaluated in a speech recognition task and compared to a conventional fragment generation approach. Results show that the proposed system produces more coherent fragments over different conditions,
which results in significantly better recognition accuracy
Most stable structure for hard spheres
The hard sphere model is known to show a liquid-solid phase transition, with
the solid expected to be either face centered cubic or hexagonal close packed.
The difference in free energy between the two structures is very small and
various attempts have been made to determine which one is the more stable. We
contrast the different approaches and extend one.Comment: 5 pages, 1 embedded figure, to appear in Phys Rev
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