D. Levesque

Perturbation theory for mixtures of simple liquids

The perturbation approach developed by Weeks, Chandler, and Andersen (WCA) and by Verlet and Weis (VW) for pure systems is here generalized to the case of mixtures. We study binary mixtures of molecules interacting with the 12–6 Lennard-Jones potential, for which Monte Carlo simulations are available for comparison. The work is divided into two parts: The first part presents results of Monte Carlo calculations on mixtures of hard spheres of 864 and 1000 particles. The radial distribution functions generated are used to test the VW representation for the correlation functions of hard-sphere mixtures. This representation is found to work satisfactorily within the expected error limits. The second part deals with the two-step perturbation procedure for calculating the thermodynamic quantities of the Lennard-Jones system. The Lennard-Jones potential is divided into a reference potential, which is strictly repulsive, and an attractive part. The system of the reference potential is represented by a system of ha...

Theoretical determination of the dielectric constant of a two dimensional dipolar fluid

Integral equations are solved to predict the pair correlation function, dielectric constant and thermodynamic properties of a two dimensional fluid. The results are compared with computer simulations including spherical truncation of the dipole potential, the reaction field method and spherical boundaries. The latter method has not yet been explored for dipolar systems and consists in placing the particles on the surface of a three dimensional sphere. Although the long range parts of the pair correlation functions predicted by these simulation methods all differ from that expected for an infinite system, the short range parts are sufficiently independent of boundary conditions and system size to permit an unambiguous test of the integral equations in this region. The most satisfactory is the QHNC equation. It gives very precise thermodynamic properties but dielectric constants which are too high compared to simulation results when the dipole moment μ* is larger then 1·5. The difference is of the order of ...

Computer simulation and theoretical results for a polar-polarizable fluid

In this paper we present molecular dynamics (MD) results for a fluid of polarizable Lennard-Jones particles with permanent dipole and quadrupole moments. Detailed comparisons are made with theoretical results obtained using a previously developed self-consistent mean field (SCMF) approximation together with the LHNC and QHNC integral equation theories. The SCMF approximation is found to give a reasonably accurate description of the polarizable system. The SCMF/QHNC and SCMF/LHNC calculations both give values for the dielectric constant and average dipole moment which are in good agreement with the MD simulations. The SCMF/QHNC theory gives better results for the pair correlation function and for some thermodynamic properties.

Molecular dynamics calculations of transport coefficients

In this article a thorough discussion is given of the various errors involved in the computation of the transport coefficient obtained by molecular dynamics simulations using the Kubo formulae. For this, the statistical error of the molecular dynamic data is directly estimated by calculating the mean square deviation of the results of several simulations performed for various thermodynamic states of systems of particles interacting through a Lennard-Jones potential or a short ranged soft-core potential. The analysis of the transverse and longitudinal correlation functions of systems of 4000 particles interacting with a soft core potential confirms that, in the immediate vicinity of the solidification line, there appear, in addition to the short lived shear modes, specific long lived ones, evocative of a solid-like behaviour.

Fluids of Lennard-Jones spheres with dipoles and tetrahedral quadrupoles

In this paper we present molecular dynamics (MD) results for fluids of Lennard-Jones particles with permanent dipole and quadrupole moments similar to those of the water molecule. We consider both the true quadrupole tensor of water and the tetrahedral quadrupole approximation which is convenient in theoretical work. The linearized and quadratic hypernettedchain (LHNC and QHNC) integral equation approximations are solved for the tetrahedral quadrupole model and detailed comparisons are made with the MD calculations. It is found that both theories are more accurate at relatively high densities with the QHNC giving better results for most fluid properties.

The potential of mean force for an infinitely dilute ionic solution

Abstract The potential of mean force between two ions immersed in a dipolar solvent is calculated in the linearized hypernetted chain approximation. The results are found to be in good agreement with Monte Carlo computations. The variation with dipole strength is shown.

Monte Carlo simulations of polar discotic molecules

The orientational order in the fluid and liquid crystal phases of a simple model of dipolar discotic molecules (cut-spheres) is determined by Monte Carlo simulations. Our results are mainly directed to a qualitative study of this ordering in view of the long relaxation times of metastable states in the dense liquid crystal phases. In the fluid and nematic phases, short columns of five molecules are found to be the most probable arrangements of neighbouring molecules. In the columnar phase the dipole-dipole interactions can induce the polarization of the columns which are arranged with antiferroelectric order. When the orientation of the dipole moment breaks the axial symmetry of the molecules, a rectangular columnar phase with molecules tilted with respect to the columnar axis appears to be the stable dense liquid crystal phase.

The potential of mean force in simple polar/non-polar mixtures

The quadratic hypernetted-chain approximation is solved for a hard sphere solute dissolved in a dipolar hard sphere solvent in the infinite dilution limit. The theory proves interesting since it yields a solute-solute potential of mean force which depends upon the dipole moment of the solvent. Significant solvophobic effects which increase with solvent dipole moment are observed. The effect of varying the solute/solvent diameter ratio is also examined.