310 research outputs found
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
Expressions for forces and torques in molecular simulations using rigid bodies
Expressions for intermolecular forces and torques, derived from pair
potentials between rigid non-spherical units, are presented. The aim is to give
compact and clear expressions, which are easily generalised, and which minimise
the risk of error in writing molecular dynamics simulation programs. It is
anticipated that these expressions will be useful in the simulation of liquid
crystalline systems, and in coarse-grained modelling of macromolecules
Helical structures from an isotropic homopolymer model
We present Monte Carlo simulation results for square-well homopolymers at a
series of bond lengths. Although the model contains only isotropic pairwise
interactions, under appropriate conditions this system shows spontaneous chiral
symmetry breaking, where the chain exists in either a left- or a right-handed
helical structure. We investigate how this behavior depends upon the ratio
between bond length and monomer radius.Comment: 10 pages, 3 figures, accepted for publication by Physical Review
Letter
Effects of three-body interactions on the structure and thermodynamics of liquid krypton
Large-scale molecular dynamics simulations are performed to predict the
structural and thermodynamic properties of liquid krypton using a potential
energy function based on the two-body potential of Aziz and Slaman plus the
triple-dipole Axilrod-Teller (AT) potential. By varying the strength of the AT
potential we study the influence of three-body contribution beyond the
triple-dipole dispersion. It is seen that the AT potential gives an overall
good description of liquid Kr, though other contributions such as higher order
three-body dispersion and exchange terms cannot be ignored.Comment: 11 pages, 3 figures, LaTeX, to appear in J. Chem. Phy
Scaling in Complex Systems: Analytical Theory of Charged Pores
In this paper we find an analytical solution of the equilibrium ion
distribution for a toroidal model of a ionic channel, using the Perfect
Screening Theorem (PST). The ions are charged hard spheres, and are treated
using a variational Mean Spherical Approximation (VMSA) .
Understanding ion channels is still a very open problem, because of the many
exquisite tuning details of real life channels. It is clear that the electric
field plays a major role in the channel behaviour, and for that reason there
has been a lot of work on simple models that are able to provide workable
theories. Recently a number of interesting papers have appeared that discuss
models in which the effect of the geometry, excluded volume and non-linear
behaviour is considered.
We present here a 3D model of ionic channels which consists of a charged,
deformable torus with a circular or elliptical cross section, which can be flat
or vertical (close to a cylinder). Extensive comparisons to MC simulations were
performed.
The new solution opens new possibilities, such as studying flexible pores,
and water phase transformations inside the pores using an approach similar to
that used on flat crystal surfaces
Electrochemical Generation of N-Heterocyclic Carbenes for Use in Synthesis and Catalysis
The electrochemical generation of N-heterocyclic carbenes (NHCs) offers a mild and selective alternative to traditional synthetic methods that usually rely on strong bases and air-sensitive materials. The use of electrons as reagents results in an efficient and clean synthesis that enables the direct use of NHCs in various applications. Herein, the use of electrogenerated NHCs in organocatalysis, synthesis and organometallic chemistry is explored
Average and extreme multi-atom Van der Waals interactions: Strong coupling of multi-atom Van der Waals interactions with covalent bonding
<p>Abstract</p> <p>Background</p> <p>The prediction of ligand binding or protein structure requires very accurate force field potentials – even small errors in force field potentials can make a 'wrong' structure (from the billions possible) more stable than the single, 'correct' one. However, despite huge efforts to optimize them, currently-used all-atom force fields are still not able, in a vast majority of cases, even to keep a protein molecule in its native conformation in the course of molecular dynamics simulations or to bring an approximate, homology-based model of protein structure closer to its native conformation.</p> <p>Results</p> <p>A strict analysis shows that a specific coupling of multi-atom Van der Waals interactions with covalent bonding can, in extreme cases, increase (or decrease) the interaction energy by about 20–40% at certain angles between the direction of interaction and the covalent bond. It is also shown that on average multi-body effects decrease the total Van der Waals energy in proportion to the square root of the electronic component of dielectric permittivity corresponding to dipole-dipole interactions at small distances, where Van der Waals interactions take place.</p> <p>Conclusion</p> <p>The study shows that currently-ignored multi-atom Van der Waals interactions can, in certain instances, lead to significant energy effects, comparable to those caused by the replacement of atoms (for instance, C by N) in conventional pairwise Van der Waals interactions.</p
Thermodynamic Models for Vapor-Liquid Equilibria of Nitrogen+Oxygen+Carbon Dioxide at Low Temperatures
For the design and optimization of CO2 recovery from alcoholic fermentation
processes by distillation, models for vapor-liquid equilibria (VLE) are needed.
Two such thermodynamic models, the Peng-Robinson equation of state (EOS) and a
model based on Henry's law constants, are proposed for the ternary mixture
N2+O2+CO2. Pure substance parameters of the Peng-Robinson EOS are taken from
the literature, whereas the binary parameters of the Van der Waals one-fluid
mixing rule are adjusted to experimental binary VLE data. The Peng-Robinson EOS
describes both binary and ternary experimental data well, except at high
pressures approaching the critical region. A molecular model is validated by
simulation using binary and ternary experimental VLE data. On the basis of this
model, the Henry's law constants of N2 and O2 in CO2 are predicted by molecular
simulation. An easy-to-use thermodynamic model, based on those Henry's law
constants, is developed to reliably describe the VLE in the CO2-rich region
Effects of Friction and Disorder on the Quasi-Static Response of Granular Solids to a Localized Force
The response to a localized force provides a sensitive test for different
models of stress transmission in granular solids. The elasto-plastic models
traditionally used by engineers have been challenged by theoretical and
experimental results which suggest a wave-like (hyperbolic) propagation of the
stress, as opposed to the elliptic equations of static elasticity. Numerical
simulations of two-dimensional granular systems subject to a localized external
force are employed to examine the nature of stress transmission in these
systems as a function of the magnitude of the applied force, the frictional
parameters and the disorder (polydispersity). The results indicate that in
large systems (typically considered by engineers), the response is close to
that predicted by isotropic elasticity whereas the response of small systems
(or when sufficiently large forces are applied) is strongly anisotropic. In the
latter case the applied force induces changes in the contact network
accompanied by frictional sliding. The larger the coefficient of static
friction, the more extended is the range of forces for which the response is
elastic and the smaller the anisotropy. Increasing the degree of polydispersity
(for the range studied, up to 25%) decreases the range of elastic response.
This article is an extension of a previously published letter [1].Comment: 21 pages (PDFLaTeX), 24 figures (some of them bitmapped to save
space); submitted to Phys. Rev.
- …