R.O. Watts

Liquid argon: Monte carlo and molecular dynamics calculations

Thermodynamic properties of liquid argon are calculated by Monte Carlo and molecular dynamics techniques, using accurate pair-potential functions determined from the properties of solid and gaseous argon, together with the Axilrod-Teller three-body interaction. Satisfactory techniques for evaluating three-body contributions to thermodynamic properties without excessive requirements of computer time are described. Quantum corrections are included. Agreement with experiment is excellent: the best pair and triplet potentials give an excellent description of the properties of solid, gaseous and liquid argon.

Intermolecular potential functions and the properties of water

Abstract A wide range of gas, liquid and solid state properties are calculated using most of the presently available potential functions for the water pair interaction. It is shown that no one model gives a satisfactory account of all three phases. We propose a new semi-empirical model that has some success as an effective pair potential in all three phases.

The structure and vibrational spectra of small clusters of water molecules

A potential surface for deformable water molecules is constructed using an accurate local-mode intramolecular model together with a realistic semi-empirical intermolecular interaction. Locally stable structures for (H 2 O) n , n = 2, 3 and 4, are reported and their vibrational frequencies calculated using both normal-mode analysis with variational corrections for anharmonicities, and local-mode analysis. It is shown that experimentally observed changes in vibrational frequencies on cluster formation arise from anharmonic terms in the monomer potential that become significant due to distortions to the equilibrium geometry. Inhomogeneous line shapes are calculated using rigid-rotor theory, linear response theory, non-equilibrium molecular dynamics and a semi-classical method based on classical Monte Carlo trajectory propagation and local-mode spectral analysis. The last method gives a satisfactory description of the density of states in the intramolecular vibrational region of the spectrum. Dipole moment functions including polarisation give infrared intensities in good agreement with available experimental data.

Monte Carlo studies of liquid water

Computer simulation studies are reported for the Rowlinson and Ben-Naim and Stillinger models of water-water interactions. Particular attention is given to the effects of altering the size of the system and to accounting for some long-range interactions by including the Onsager reaction field. It is shown that both models give a good qualitative account of the structure of liquid water but that neither is able to describe the high dielectric constant. A particularly sensitive property, the dipole-dipole correlation function, demonstrates the problems encountered in truncating the water interactions. Good agreement between the Rowlinson potential and a modified Hartree-Fock calculation suggests that the Rowlinson model is more accurate than the Ben-Naim and Stillinger form.

The structure, thermodynamic properties and infrared spectra of liquid water and ice

Abstract Monte Carlo simulations are used, together with models of the intramolecular and intermolecular potential surfaces, to model liquid water and several phases of ice. Intramolecular relaxation makes important contributions to both thermodynamic and structural properties. A quantum local mode analysis of the Monte Carlo configurations is used to predict the density of states and infrared absorption intensities for the intramolecular bending and stretching vibrations. The large shifts from the gas phase Oue5f8H stretch frequencies observed experimentally in the liquid and solid phases are due to anharmonic terms in the intramolecular surface rather than to harmonic intermolecular coupling. A significant contribution to observed changes in IR intensity on condensation arises from the large molecular polarisability.

A local mode potential function for the water molecule

The molecular rotation-vibration hamiltonian is written in local coordinates and a variational technique for calculating accurate eigenfunctions and eigenvalues is described. It is shown that the local mode approach is rapidly convergent and no more difficult to apply than the normal mode method. A simple four-parameter Morse potential function is fitted to 56 vibrational levels of H2O, HDO and D2O, and used to predict all ro-vibrational levels of up to 5 vibrational quanta with J = 0, 1 and 2. The agreement between calculated and observed frequencies is remarkably good for a model containing so few empirical parameters.

On the structure of liquid benzene

Monte Carlo calculations of the structure of liquid benzene are reported. The benzene pair potential is represented as a sum of Lennard-Jones (12, 6) interactions acting between six sites on each molecule, giving a total of 36 terms. Results are given for the centre-of-mass radial distribution function, for CH group/CH group distributions and for the centre-of-mass/CH group distribution function. Angular correlation functions are examined in terms of the axial symmetry vector of the benzene ring. It is shown that the liquid-state structure resembles that of the crystalline solid, and in particular that the nearest-neighbour distribution can be interpreted using this similarity.

Rotational structure in the infrared spectra of carbon dioxide and nitrous oxide dimers

Abstract High-resolution infrared predissociation spectra have been measured for dilute mixtures of CO 2 and N 2 O in helium. Rotational fine structure is clearly resolved for both (CO 2 ) 2 and (N 2 O) 2 , the linewidths being instrument-limited. This establishes that predissociation lifetimes are longer than approximately 50 ns.

Vibrational predissociation spectra of nitrous oxide clusters

Abstract A colour-centre laser has been used in conjunction with a molecular-beam apparatus to study the infrared vibrational predissociation spectra of nitrous oxide clusters near the ν1 + ν3 vibrational combination band of the momomer. These spectra were recorded as a function of source pressure in order to measure their dependence on cluster size. A frequency shift, corresponding to a gradual transition from the vapour to the solid state, is observed as the cluster size increases. The cluster-size distributions were estimated using both quadrupole and ion time-of-flight mass spectrometry. The latter technique shows cluster sizes well in excess of (N2O)100. Semiclassical studies based on Monte Carlo simulations and local mode analysis of representative cluster configurations are used to account for line profiles and shifts.

Interactions between benzene molecules

A number of semi-empirical potentials are presented for benzene interactions that include both two-body and three-body effects. The two-body interaction gives accurate second virial coefficients for benzene vapour and when combined with a three-body term gives a good account of the static lattice energy and lattice parameters of the zero temperature crystal. A formula for the three-body quadrupole-induction energy is also derived.