15,659 research outputs found
Quasichemical theory and the description of associating fluids relative to a reference: Multiple bonding of a single site solute
We derive an expression for the chemical potential of an associating solute
in a solvent relative to the value in a reference fluid using the quasichemical
organization of the potential distribution theorem. The fraction of times the
solute is not associated with the solvent, the monomer fraction, is expressed
in terms of (a) the statistics of occupancy of the solvent around the solute in
the reference fluid and (b) the Widom factors that arise because of turning on
solute-solvent association. Assuming pair-additivity, we expand the Widom
factor into a product of Mayer f-functions and the resulting expression is
rearranged to reveal a form of the monomer fraction that is analogous to that
used within the statistical associating fluid theory (SAFT). The present
formulation avoids all graph-theoretic arguments and provides a fresh, more
intuitive, perspective on Wertheim's theory and SAFT. Importantly, multi-body
effects are transparently incorporated into the very foundations of the theory.
We illustrate the generality of the present approach by considering examples of
multiple solvent association to a colloid solute with bonding domains that
range from a small patch on the sphere, a Janus particle, and a solute whose
entire surface is available for association
Mini-grand canonical ensemble: chemical potential in the solvation shell
Quantifying the statistics of occupancy of solvent molecules in the vicinity
of solutes is central to our understanding of solvation phenomena. Number
fluctuations in small `solvation shells' around solutes cannot be described
within the macroscopic grand canonical framework using a single chemical
potential that represents the solvent `bath'. In this communication, we
hypothesize that molecular-sized observation volumes such as solvation shells
are best described by coupling the solvation shell with a mixture of particle
baths each with its own chemical potential. We confirm our hypotheses by
studying the enhanced fluctuations in the occupancy statistics of hard sphere
solvent particles around a distinguished hard sphere solute particle.
Connections with established theories of solvation are also discussed
Experimental observation of chimera and cluster states in a minimal globally coupled network
A "chimera state" is a dynamical pattern that occurs in a network of coupled
identical oscillators when the symmetry of the oscillator population is broken
into synchronous and asynchronous parts. We report the experimental observation
of chimera and cluster states in a network of four globally coupled chaotic
opto-electronic oscillators. This is the minimal network that can support
chimera states, and our study provides new insight into the fundamental
mechanisms underlying their formation. We use a unified approach to determine
the stability of all the observed partially synchronous patterns, highlighting
the close relationship between chimera and cluster states as belonging to the
broader phenomenon of partial synchronization. Our approach is general in terms
of network size and connectivity. We also find that chimera states often appear
in regions of multistability between global, cluster, and desynchronized
states
Large-scale exact diagonalizations reveal low-momentum scales of nuclei
Ab initio methods aim to solve the nuclear many-body problem with controlled
approximations. Virtually exact numerical solutions for realistic interactions
can only be obtained for certain special cases such as few-nucleon systems.
Here we extend the reach of exact diagonalization methods to handle model
spaces with dimension exceeding on a single compute node. This allows
us to perform no-core shell model (NCSM) calculations for 6Li in model spaces
up to and to reveal the 4He+d halo structure of this
nucleus. Still, the use of a finite harmonic-oscillator basis implies
truncations in both infrared (IR) and ultraviolet (UV) length scales. These
truncations impose finite-size corrections on observables computed in this
basis. We perform IR extrapolations of energies and radii computed in the NCSM
and with the coupled-cluster method at several fixed UV cutoffs. It is shown
that this strategy enables information gain also from data that is not fully UV
converged. IR extrapolations improve the accuracy of relevant bound-state
observables for a range of UV cutoffs, thus making them profitable tools. We
relate the momentum scale that governs the exponential IR convergence to the
threshold energy for the first open decay channel. Using large-scale NCSM
calculations we numerically verify this small-momentum scale of finite nuclei.Comment: Minor revisions.Accepted for publication in Physical Review
Structure and thermodynamics of a mixture of patchy and spherical colloids: a multi-body association theory with complete reference fluid information
A mixture of solvent particles with short-range, directional interactions and
solute particles with short-range, isotropic interactions that can bond
multiple times is of fundamental interest in understanding liquids and
colloidal mixtures. Because of multi-body correlations predicting the structure
and thermodynamics of such systems remains a challenge. Earlier Marshall and
Chapman developed a theory wherein association effects due to interactions
multiply the partition function for clustering of particles in a reference
hard-sphere system. The multi-body effects are incorporated in the clustering
process, which in their work was obtained in the absence of the bulk medium.
The bulk solvent effects were then modeled approximately within a second order
perturbation approach. However, their approach is inadequate at high densities
and for large association strengths. Based on the idea that the clustering of
solvent in a defined coordination volume around the solute is related to
occupancy statistics in that defined coordination volume, we develop an
approach to incorporate the complete information about hard-sphere clustering
in a bulk solvent at the density of interest. The occupancy probabilities are
obtained from enhanced sampling simulations but we also develop a concise
parametric form to model these probabilities using the quasichemical theory of
solutions. We show that incorporating the complete reference information
results in an approach that can predict the bonding state and thermodynamics of
the colloidal solute for a wide range of system conditions.Comment: arXiv admin note: text overlap with arXiv:1601.0438
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