10,043 research outputs found
Quasi-chemical theory with a soft cutoff
In view of the wide success of molecular quasi-chemical theory of liquids,
this paper develops the soft-cutoff version of that theory. This development
has important practical consequences in the common cases that the packing
contribution dominates the solvation free energy of realistically-modeled
molecules because treatment of hard-core interactions usually requires special
purpose simulation methods. In contrast, treatment of smooth repulsive
interactions is typically straightforward on the basis of widely available
software. This development also shows how fluids composed of molecules with
smooth repulsive interactions can be treated analogously to the molecular-field
theory of the hard-sphere fluid. In the treatment of liquid water,
quasi-chemical theory with soft-cutoff conditioning doesn't change the
fundamental convergence characteristics of the theory using hard-cutoff
conditioning. In fact, hard cutoffs are found here to work better than softer
ones.Comment: 5 pages, 2 figure
Balancing Local Order and Long-Ranged Interactions in the Molecular Theory of Liquid Water
A molecular theory of liquid water is identified and studied on the basis of
computer simulation of the TIP3P model of liquid water. This theory would be
exact for models of liquid water in which the intermolecular interactions
vanish outside a finite spatial range, and therefore provides a precise
analysis tool for investigating the effects of longer-ranged intermolecular
interactions. We show how local order can be introduced through quasi-chemical
theory. Long-ranged interactions are characterized generally by a conditional
distribution of binding energies, and this formulation is interpreted as a
regularization of the primitive statistical thermodynamic problem. These
binding-energy distributions for liquid water are observed to be unimodal. The
gaussian approximation proposed is remarkably successful in predicting the
Gibbs free energy and the molar entropy of liquid water, as judged by
comparison with numerically exact results. The remaining discrepancies are
subtle quantitative problems that do have significant consequences for the
thermodynamic properties that distinguish water from many other liquids. The
basic subtlety of liquid water is found then in the competition of several
effects which must be quantitatively balanced for realistic results.Comment: 8 pages, 6 figure
Final state interactions in two-particle interferometry
We reconsider the influence of two-particle final state interactions (FSI) on
two-particle Bose-Einstein interferometry. We concentrate in particular on the
problem of particle emission at different times. Assuming chaoticity of the
source, we derive a new general expression for the symmetrized two-particle
cross section. We discuss the approximations needed to derive from the general
result the Koonin-Pratt formula. Introducing a less stringent version of the
so-called smoothness approximation we also derive a more accurate formula. It
can be implemented into classical event generators and allows to calculate FSI
corrected two-particle correlation functions via modified Bose-Einstein
"weights".Comment: 12 pages RevTeX, 2 ps-figures included, submitted to Phys. Rev.
Quantum corrections for pion correlations involving resonance decays
A method is presented to include quantum corrections into the calculation of
two-pion correlations for the case where particles originate from resonance
decays. The technique uses classical information regarding the space-time
points at which resonances are created. By evaluating a simple thermal model,
the method is compared to semiclassical techniques that assume exponential
decaying resonances moving along classical trajectories. Significant
improvements are noted when the resonance widths are broad as compared to the
temperature.Comment: 9 pages, 4 figure
Effect of the source charge on charged-beam interferometry
We investigate quantal perturbations of the interferometric correlations of
charged bosons by the Coulomb field of an instantaneous, charged source. The
source charge increases the apparent source size by weakening the correlation
at non-zero relative momenta. The effect is strongest for pairs with a small
total momentum and is stronger for kaons than for pions of the same momenta.
The experimental data currently available are well described by this effect
without invoking Pratt's exploding source model. A simple expression is
proposed to account for the effect.Comment: 9 pages TEX, 3 Postscript figures available at
http://www.krl.caltech.edu/preprints/MAP.htm
Covariance of Antiproton Yield and Source Size in Nuclear Collisions
We confront for the first time the widely-held belief that combined
event-by-event information from quark gluon plasma signals can reduce the
ambiguity of the individual signals. We illustrate specifically how the
measured antiproton yield combined with the information from pion-pion HBT
correlations can be used to identify novel event classes.Comment: 8 pages, 5 figures, improved title, references and readability;
results unchange
Canonical and Microcanonical Distributions for Fermi Systems
Recursion relations are presented that allow exact calculation of canonical
and microcanonical partition functions of degenerate Fermi systems, assuming no
explicit two-body interactions. Calculations of the level density, sorted by
angular momentum, are presented for Ni-56 are presented. The issue of treating
unbound states is also addressed.Comment: 5 pages, 5 figure
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