Ian R. McDonald

Molecular‐dynamics simulation of aqueous mixtures: Methanol, acetone, and ammonia

Effective pair‐potential models, parametrized to the properties of the pure liquids, have been used in molecular‐dynamics simulations of aqueous (binary) mixtures containing methanol, ammonia, or acetone. Results are reported for thermodynamic and structural properties, self‐diffusion coefficients, and reorientational correlation times. There is fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen‐bond lifetimes in the simulated mixtures have been investigated. The observed anomalous behavior of methanol and acetone solutions appears to be related to specific features of the hydrogen bonding—namely, the ability of these molecules to exhibit enhanced acceptor character. As a consequence of the assumed intermolecular potentials, the balance between the competing effects of hydrophobic hydration of methyl groups and hydrogen bonding to oxygen atoms is tipped towards the latter. A number of interesting structural effects have been noted. In ...

Spectroscopic and transport properties of water

Computer simulation by the method of molecular dynamics has been used to study the dynamical properties of liquid water at several state points. The interactions between molecules were represented by the potential energy function of Matsuoka, Clementi and Yoshimine. The main effort has been directed at extracting from the simulations results of significance for the interpretation of measurements of spectroscopic and transport properties of real water, including isotope effects. Results are reported for the self diffusion coefficient, N.M.R. relaxation times, infrared and Raman spectra, and functions relevant to coherent and incoherent inelastic neutron scattering and to dielectric relaxation. A basis is provided for the explanation of a number of experimental observations.

Interaction site models for carbon dioxide

The adequacy of simple potential models to describe the properties of carbon dioxide in its different phases is explored. The molecules are assumed to interact through two or three Lennard-Jones n-6 centres and a point quadrupole moment placed at the molecular centre of mass. Good agreement with a wide range of properties is obtained for a three-centre model with n=12 and a value of the quadrupole moment which is somewhat smaller than the experimental one.

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.

Pairwise additive effective potentials for nitrogen

A number of pairwise additive intermolecular potentials have been used in the study of properties of nitrogen in the solid, liquid and gas phases. It is shown that a simple model can account satisfactorily for a wide range of phenomena.

An effective pair potential for liquid acetonitrile

A six site potential model has been developed for liquid acetonitrile, based on atomic interaction parameters taken from studies of other liquids and partial charges derived by ab initio calculations. When used in molecular dynamics simulations, the model yields results in good agreement with experimental data on the thermodynamics, structure, self diffusion coefficients and reorientational correlation times of the liquid.

A computer simulation study of the dielectric properties of a model of methyl cyanide

The dielectric permittivity of a model fluid representing methyl cyanide has been studied with a view to testing a recently developed theory of the frequency dependent dielectric permittivity. Although there are large correlations between the orientations of dipoles the Kirkwood g factor is close to the trivial value of unity; this contrasts with the behaviour of Stockmayer and similar model fluids and is a consequence of the molecular shape. The time dependence of both the longitudinal and transverse components of the dipole density is obtained and related to the relaxation of the single molecule dipole correlation function. The applicability of the ‘dipolaron’ description to the oscillation observed in the relaxation of the longitudinal component is examined.

Molecular dynamics calculations for the liquid and cubic plastic crystal phases of carbon tetrachloride

Molecular dynamics calculations on liquid carbon tetrachloride have been used to parameterize atom-atom intermolecular potentials. The resulting models have been used to study the structure and dynamics of the face centred cubic plastic crystal, phase Ia.

Molecular dynamics studies of the structure of water at high temperatures and density

The effect of changes in temperature and density on the structure of water has been studied by molecular dynamics using the analytical potential of Matsuoka, Clementi, and Yoshimine. Significant structural changes are found to occur, particularly at high density. Implications for the interpretation of neutron scattering experiments are discussed.

Disorder in the pseudohexagonal rotator phase of n-alkanes: molecular-dynamics calculations for tricosane

Molecular-dynamics calculations for a flexible-chain model have been used to study two, solid, bilayer phases of the n-alkane tricosane (C23H48). In the crystalline, orthorhombic phase at 311 K, the chains remain all-trans and fully ordered, with a herringbone packing. By contrast, in the pseudohexagonal (R I) rotator phase at about 320 K, a dramatic increase in longitudinal chain motion is observed, each chain has four well-defined orientations, and a significant number of conformational defects develop, with the latter occurring predominantly at the chain ends. The results of the simulations are in excellent agreement with experiment.