Seamus F. OShea

Electrostatic interactions in molecular crystals

Lattice dynamics calculations for nitrogen and carbon dioxide have been carried out for empirical intermolecular potentials which incorporate accurate representations of the molecular charge distributions. In the case of nitrogen, the higher order moments are found to make large negative contributions to the harmonic librational frequencies. The differences between nitrogen and carbon dioxide and the implications for other crystals are discussed.

Vapour-liquid equilibria for two centre Lennard-Jones diatomics and dipolar diatomics

Gibbs ensemble Monte Carlo simulations have been used to calculate vapour-liquid equilibria of diatomic and dipolar diatomic fluids. Dipolar two-centre Lennard-Jones diatomics with elongations L* = L/σ between 0·33 and 1·00 have been studied for various values of dipole moments μ*d = μd/(eσ3)1/2. The results for phase equilibrium are in good agreement with recent theoretical predictions based on cluster-expansion perturbation theory.

Orientational phases of classical quadrupoles on a triangular net

Abstract The classical Monte Carlo (MC) method is used to study the effect of temperature on the ordering of quadrupoles arranged on a triangular net with toroidal boundary conditions. The results are used to discuss the orientational ordering of nitrogen and hydrogen molecules adsorbed on the basal plane of graphite.

Gas–surface potentials and the dynamics of overlayers

Lattice dynamics (LD) and molecular dynamics (MD) calculations have been used to investigate phonons propagating in monolayers, bilayers and trilayers of rare gases physisorbed on the Ag(111) surface. Adatom–surface potentials for the monolayers are represented by a Morse function whose well depth and curvature are parametrized to reproduce the latent heat of condensation and the frequency of the dispersionless phonon branch with atomic displacements perpendicular to the surface. In the bilayers and trilayers the second and third overlayers are assumed to interact with the surface via the known van der Waals terms. Realistic potentials are used to represent the interactions between the adatoms. If the interlayer separations are optimized with due allowance for thermal dilation effects the LD results are in agreement both with the MD calculations and with the available experimental data for the bilayers and trilayers. The sensitivity of the calculations to variations in the parameters of the model potential has been investigated.

Fast, portable, and reliable algorithm for the calculation of Halton numbers☆

Abstract We give a recursive algorithm closely based on the definition of Halton numbers— reflection in the radical point of the digits of an integer in an arbitrary-base positional notation—which is, unlike the Haltons short algorithm, not at all affected by the round-off error, and much faster than the recent improvement of the Haltons algorithm by Berblinger and Schlier. Some applications of the Halton numbers are discussed.

Simulation of finite patches of xenon on graphite

Abstract Finite patches of xenon adsorbed on the basal plane of graphite are used to model solid adsorbed xenon in the low coverage regime. Molecular dynamics simulations at constant temperature of patches of 10 2 -10 3 atoms with free boundaries reveal a rich phenomenology. Evidence is found of a commensurate-incommensurate transition at 30 K which produces a phase of hexagonal domains, typically of about 90 atoms, with mobile walls, and a transition at 60 K which rotates the patch out of alignment with the graphite substrate. The patch as a whole is observed to exhibit large orientational fluctuations on a timescale of tens of picoseconds, and individual domains within the patch have some rotational freedom. At high temperatures the patches develop voids along the domain boundaries, and the resulting small crystallites evaporate to a two-dimensional fluid.

Analytical approximations to virial coefficients for pure and mixed systems

Analytic approximations, in the form of finite polynomial expansions in T -1, are given to the pressure virial coefficients of pure Lennard-Jones systems with strictly pairwise-additive potentials. Results are given through fifth order in both two and three dimensions. These analytic forms are combined in Pade approximants for the compressibility factor in three dimensions, and the critical constants found from them are in agreement with those obtained elsewhere; equally good agreement is found in the two dimensional case. Also reported are calculations of two- and three-dimensional third virial coefficients of mixtures of the heavier rare gases and simple molecules. The interactions are represented by single centre models, Lennard-Jones potentials which are shown to give mixed second virial coefficients in reasonable agreement with experimental values. The calculated and experimental third virials do not agree, but the inclusion of three-body interactions, through the triple-dipole and related terms, imp...

Vibrational amplitudes of xenon overlayers physisorbed on the graphite basal plane

Abstract We have carried out classical Monte Carlo calculations for xenon overlayers physisorbed on the graphite basal plane. The density profile of the xenon atoms normal to the graphite surface has been studied as a function of temperature for an epitaxial monolayer and a bilayer. Comparison is made with molecular beam and electron diffraction data and with lattice-dynamics calculations.

Hard discs with embedded three dimensional quadrupoles

A survey of the structural and thermodynamic behaviour of hard discs with embedded three dimensional quadrupoles of linear symmetry has been made using Monte Carlo methods. The results for dilute samples at high temperatures (small quadrupole moments) are consistent with thermodynamic perturbation theory. Those for higher density or lower temperatures (larger quadrupole moments) indicate the existence of two solid phases, one a high density hexagonal solid and the other a lower density square structure. No convincing evidence was found for the existence of a distinct liquid phase, but the data are not sufficiently detailed to provide a reliable determination of the phase diagram. Structural evidence has been found for the coexistence of the two solids, and the interface between the two structures seems to lead to relatively little disruption in either phase, at least for small ‘domain’ sizes. The stability of the square structure is easily understood in terms of the large quadrupole energy it achieves, in...

Structure of liquid metals. Monte Carlo and integral equation perturbation theory

In an effort to understand the effectiveness of the zero-order blip function theory, Monte Carlo computer simulations were carried out for the repulsive portion of pair interaction potentials for several liquid metals. The results are compared with integral equation perturbation theory using Percus-Yevick (PY) and hypernetted chain (HNC) equations. It has been found that for smooth repulsive potentials PY integral equation perturbation theory yields structure in good agreement with computer simulation results.