J. J. Weis

Perturbation theories for polar fluids

The free energy of dense fluids with permanent dipoles is computed through the Monte Carlo method with two types of intermolecular interaction: Stockmayer potential and hard-sphere potential with an added dipolar interaction. A comparison is made with the Pade representation recently proposed by Stell which is shown to be good. In the case of hard spheres with permanent dipoles we propose another equally successful perturbation theory, which is based on a partial resummation of the mixed perturbation theory first introduced by Lebowitz, Stell, and Baer. This perturbation theory is then applied to the general case of an interaction composed of a spherically symmetric two-body potential with permanent dipoles.

Perturbation theory for the thermodynamic properties of simple liquids

The perturbation theory of simple liquids has recently met with great success. In this paper we present a critical discussion of this theory. The unperturbed system is, as usual, identified with hard spheres whose diameters are determined by a self-consistency requirement. The thermodynamics of the system, including the first-order contribution, can be computed analytically. The equation of state thus obtained is used to discuss experimental results on non-polar liquids.

Charged hard spheres in dipolar hard sphere solvents. A model for electrolyte solutions

The linearized and quadratic hypernetted‐chain approximations are solved numerically for mixtures of charged hard spheres in dipolar hard sphere solvents. The pair correlation functions, concentration dependent dielectric constant, and thermodynamic properties are examined for several solvents and a range of ion concentrations. The ion–ion pair correlation functions contain many interesting structural features which are not present in the so‐called primitive model. Solvent separated ion pairs are found to be a very important species in certain solutions.

A Monte Carlo study of the classical two-dimensional one-component plasma

We present results from extensive Monte Carlo simulations of the fluid phase of the two-dimensional classical one-component plasma (OCP). The difficulties associated with the infinite range of the logarithmic Coulomb interaction are eliminated by confining the particles to the surface of a sphere. The results are compared to those obtained for a planar system with screened Coulomb interactions and periodic boundary conditions; in this case the infinite tail of the Coulomb interaction is treated as a perturbation. The “exact” simulation results are used to test various approximate theories, including a semiempirical modification of the hypernetted-chain (HNC) integral equation. The OCP freezing transition is located at a couplingγ= e2/kBT−140.

A Monte Carlo study of dipolar hard spheres The pair distribution function and the dielectric constant

This paper is a systematic investigation of the effects of boundary conditions upon Monte Carlo calculations for dipolar fluids. Results are reported for the minimum image (MI), spherical cut-off (SC) and uniform reaction field methods. All three approximations are shown to give different pair distribution functions, g(12), and none yields the infinite system result. It is concluded that theories giving g(12) for an infinite system should not be evaluated by direct comparison with Monte Carlo results. Two alternative methods by which meaningful comparisons can be made are described in the text. The dependence of the thermodynamic properties upon boundary conditions is important only at large values of the dipole moment. For small to moderate dipoles both MI and SC are found to give reasonable estimates of the dielectric constant.

Theoretical calculation of ionic solution properties

A model system of electrolyte solution is studied by molecular dynamics simulation. The results show how the polarizability of the molecules and the ratio of the molecular diameters of the ions and solvent molecules affect the properties of the system. The computation of the frequency dependent dielectric constant and conductivity in terms of correlation functions of the electrical current and microscopic polarization is discussed. A general solution of this problem is given for systems of arbitrary shape composed of nonpolarizable ions and solvent molecules. Three particular cases are considered in detail: the infinite system; a spherical system in contact with a dielectric and conducting continuum; a system with periodic boundary conditions. The zero frequency limit of the dielectric constant and conductivity is investigated.

Integral equation approximations for dipolar fluids

The linearized and quadratic hypernetted-chain approximations are solved for fluids of dipolar hard spheres and Stockmayer particles. The density and dipole moment dependence of the pair correlation function, dielectric constant and some thermodynamic properties is investigated. Comparisons with new Monte Carlo results show that, at low density, the linearized theory gives a very poor description of the pair correlations. The quadratic approximation, however, is found to be excellent for all densities and dipole moments considered.

Density dependence of the dephasing and energy relaxation times by computer simulations

The energy relaxation time (T1) and dephasing time (T2) of a system of molecules interacting by spherically symmetric intermolecular and vibrational potentials have been evaluated by the molecular dynamics method. The computational ease introduced by the spherical symmetry of the potentials allows an extensive study of the density variations of T1 and T2 from the gas phase to the highly compressed liquid and solid, as well as of the influence of a long range component in the vibrational potential on the relaxation times. The general trends which proceed from this study are compared with recent theoretical approaches and experimental observations.

Correlation functions of hard sphere fluids adsorbed in porous media

We report Monte Carlo (MC) simulations for the pair correlation functions of a hard sphere fluid in a hard sphere matrix. In particular, we evaluate the blocking function hb(r) which is that part of the fluid pair correlation function which accounts for the correlation between fluid particles mediated through at least one matrix particle. The results are compared with integral equation theory. A scheme for determining the bridge functions is presented which predicts excellent agreement of the pair structure with MC results.

A molecular dynamics simulation of dephasing in liquid nitrogen. II. Effect of the pair potential on dephasing

We carry out a computer simulation of vibrational dephasing in liquid nitrogen near its boiling point using a multiparameter potential which includes short range valence, dispersion, and quadrupole interactions. The effect of vibration–rotation coupling on the line shape is also included in the calculation. The isotropic Raman line shift obtained is −5 cm−1 while the dephasing time T2 (inversely proportional to the linewidth) is 100±10 psec. It is shown that there is substantial cancellation between repulsive and attractive contributions to the linewidths, so that the Raman spectrum is very sensitive to details of the molecular interactions.