David J. Wales

Melting and freezing of small argon clusters

An investigation of melting, freezing, and coexistence phenomena is presented for small clusters using Ar7, Ar8, Ar13, and Ar14 as specific representative examples. Combining the results of molecular dynamics simulations, especially short‐time kinetic energy averages and quenching, with accurate calculations of the local minima and transition states illuminates the relationship between the potential energy surface and dynamical processes. The results are consistent with a recent general defect theory of melting.

Finding saddle points for clusters

The Cerjan–Miller eigenvector‐following method is highly successful in finding saddle points for a variety of clusters whose potential energy surfaces are well described using simple analytical pair potentials. Examples are given for argon clusters, including Ar33 and Ar55, potassium chloride clusters and clusters of ions held in an ‘‘ion trap.’’ The method reveals a number of important patterns and opens the way to a more detailed understanding of the relation between energetics, reactivity, and potential energy surfaces for these and other systems.

Freezing, melting, spinodals, and clusters

A simple theory for melting, freezing, and phase coexistence is presented with emphasis on the relation between small clusters and spinodal behavior. This theory, coupled with the results of computer simulations, suggests a reinterpretation of spinodals and first‐order phase changes. We find sufficient conditions for a system of mutually attracting quasiparticles to exhibit the characteristic ‘‘S‐bend’’ found, e.g., for subcritical isotherms of a van der Waals fluid, and discuss how the theory may be extended explicitly to finite clusters.

Exploring potential energy surfaces with transition state calculations

Means are presented for using stationary points in two ways. One, for well‐understood potentials, elucidates relations between the form of the surface and the dynamics that it supports, including the determination of the effective molecular symmetry group. The other, for potentials of uncertain quality, provides a test for unphysical characteristics and suggests how the surface might be improved if it is found to be unsatisfactory in some respect. Our approach involves comparison of transition state calculations using the slowest slide and Cerjan–Miller algorithms for two example systems: the Lennard‐Jones Ar7 cluster and the Handy–Carter many‐body‐expansion potential for the ground state of formaldehyde.

Transition states for Ar55

Abstract Five transition states for the Mackay icosahedral Ar 55 cluster have been calculated, demonstrating the power of the Cerjan-Miller eigenvector-following method when applied to such systems. These results represent an important step towards a more detailed understanding of the relation between energetics, reactivity and the potential energy surfaces for these and related systems.

Topography of potential-energy surfaces for Van der Waals complexes

The recently developed program ORIENT 3 is applied to several complexes including acetonitrile dimer, benzene–water, benzene–ammonia and chlorine dimer. We employ distributed multipole analysis in conjunction with the eigenvector-following optimization technique to calculate minima, saddle points and rearrangement pathways. The potential-energy surface of the chlorine dimer is investigated as an example of anisotropic repulsion.

Structure and growth of colloidal metal particles

In this paper we discuss the results of electron microscopic studies of colloidal gold and silver metal particles in the context of a model for their structure and growth. The images obtained are compared with the results of computer simulations for various candidate structures and the actual morphologies present are thereby deduced. With reference to earlier work on inert gas atom clusters we have developed a scheme which models the growth of such particles and enables the structures present to be predicted and rationalized. In particular, we deduce that particles belonging to the icosahedral point group have Mackay‐type structures, in agreement with the experimental results. Comparisons are also made between the morphologies adopted by gold, silver, and argon clusters, and the relationship between the bonding in these systems is discussed.

Skeletal rearrangements in clusters. III: Application of vibrational symmetry analyses

Abstract We show how geometrical selection rules (such as those due to McIvor and Stanton) may be applied to cluster rearrangements to provide further insight into the types of process which may occur. We also demonstrate how a second-order Jahn—Teller vibrational analysis fits in with the orbital symmetry selection rules for diamond—square—diamond processes.