David G. Bounds

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.

Intermolecular potentials for ammonia based on SCF–MO calculations

The results of SCF molecular orbital calculations on the ammonia dimer have been used in part to parameterize a set of atom–atom potentials. When combined with a charge distribution which reproduces the experimental dipole and quadrupole moments of the monomer, and with independent estimates of the dispersion energy, the resulting intermolecular potential yields a fair description of certain properties of the condensed phases of ammonia. Liquid ammonia is predicted to have a weakly associated character.

An abinitio molecular orbital study of NaCN and KCN

Ab initio self‐consistent‐field calculations using large flexible Gaussian basis sets are reported for sodium and potassium cyanide. The potential energy curves calculated for the linear cyanide, linear isocyanide, and a T‐shaped geometry in which Na+ or K+ approaches the center of mass of a rigid (CN)− indicate that the T‐ shaped minimum is lowest in energy and the linear cyanide structure highest. The fully optimized equilibrium (re) geometries determined for both complexes are significantly nonlinear. Nearly L‐shaped structures KNC and NaNC lie ∼1.7 and ∼0.8 kcal/mol lower than the optimized linear isocyanide geometries. For KCN, the optimum geometry is calculated to be re(KC) = 2.850 A, re(KN) = 2.616 A, re(CN) = 1.153 A, ϑe(KCN) = 66.6°, and ϑe(KNC) = 90.5° in good agreement with recent experimental determinations of the gas phase structure. The predicted geometry for NaCN is re(NaC) = 2.470 A, re(NaN) = 2.195 A, re(CN) = 1.152 A, ϑe(NaCN) = 62.7°, and ϑe(NaNC) = 89.5°. The electronic distributions i...

Molecular dynamics simulation of the plastic phase of solid methane

A molecular dynamics (MD) calculation has been carried out for a model of solid methane at a state condition close to the triple point. We have used systems of 32 and 108 molecules interacting via atom–atom potentials. Equilibrium properties such as the configurational energy, pressure, and specific heat have been evaluated and compared where possible with experimental data. The simulated static structure factor has been analyzed assuming that rotational and translational motion can be decoupled and that the rotational structure factor can be developed as a power series in Kubic harmonics. This description is shown to yield a reasonably faithful representation of the structure in the plastic phase. The dynamical structure factor S(Q,ω) has been calculated for selected values of the momentum transfer h/Q. These results have been used to estimate the velocity of sound and hence the elastic constants. The overall agreement of the dynamical properties with experiment is not particularly good, indicating the need for a more refined intermolecular potential.

The convergence of the energy cluster expansion in Li n Cl(n-1)+

Non-additivity in the energies of Li n Cl(n-1)+ ion clusters (n = 2→4) has been studied by means of ab initio SCF calculations. Four-body energies contribute less than 2 per cent to the total interaction energies, while in the only Li4Cl3+ structure considered the five-body energy is negligibly small. However three-body energies are not small but are similar in size to the non-coulomb two-body energies. We have investigated to what extent these non-additive effects can be explained in terms of a simple polarizable ion model.

The shapes of pair polarizability curves

The relationship between the shapes of the pair polarizability curves of diatomic systems and chemical bonding is explored by means of molecular orbital calculations. The pair polarizability of a diatomic molecule can be split up into contributions from individual MO’s. This analysis shows that β(2)(R), like chemical bonding, is essentially determined by valence shell orbitals; hence the shapes of β(2)(R) curves for various pairs of interacting atoms can be related to the shapes of their potential curves. The relationship between β(2)(R) and molecular shape is illustrated for the system of a point positive charge and a chloride ion.

The interaction energies and polarizabilities of sodium fluoride, sodium chloride, and some alkali and halide ions pairs

Abstract Ab initio SCF pair potentials and polarizabilities for NaF, NaCl, F 2− 2 , Na 2− 2 , K 2− 2 , FCl 2− , LiNa 2+ , LiK 2+ , presented. Together with results reported previously, these values form a complete and consistent set of energy and polarizability data on the fluorides and chlorides of lithium, sodium and potassium.

Structure and dynamics of associated molecular systems. IV. The orientationally disordered phase I of solid DCl

Two recently proposed models for the intermolecular interaction between hydrogen chloride molecules have been used in a molecular dynamics simulation of the plastic crystal phase I of DCl. The simulated static structure factors are compared with neutron scattering data. Of the two models considered, only that which incorporates an explicit hydrogen bond interaction is able to account for the observed structure. For this model, examination of the deuteron motion reveals that rotational diffusion proceeds predominantly via 60 ° flips of the molecules. The dynamical structure factor has been calculated for selected values of the momentum transfer together with the power spectra associated with the autocorrelation functions of the center of mass and angular velocities. The results are discussed in the light of inelastic neutron scattering data.

Static disorder in the mixed crystals (KCN) x (KBr)1−x and its relation to dynamical properties

The dynamical properties of the mixed crystal (KCN) x (KBr)1−x and the static disorder of the CN− ions have been studied by molecular dynamics calculations. Particular attention is given to the effect of cooling on the behaviour of the CN− ions and evidence is found of the formation of an orientational glass state at low temperatures. The results are in satisfactory agreement with those of neutron inelastic scattering experiments.