C. L. Cleveland

The energetics and structure of nickel clusters: Size dependence

The energetics of nickel clusters over a broad size range are explored within the context of the many‐body potentials obtained via the embedded atom method. Unconstrained local minimum energy configurations are found for single crystal clusters consisting of various truncations of the cube or octahedron, with and without (110) faces, as well as some monotwinnings of these. We also examine multitwinned structures such as icosahedra and various truncations of the decahedron, such as those of Ino and Marks. These clusters range in size from 142 to over 5000 atoms. As in most such previous studies, such as those on Lennard‐Jones systems, we find that icosahedral clusters are favored for the smallest cluster sizes and that Marks’ decahedra are favored for intermediate sizes (all our atomic systems larger than about 2300 atoms). Of course very large clusters will be single crystal face‐centered‐cubic (fcc) polyhedra: the onset of optimally stable single‐crystal nickel clusters is estimated to occur at 17 000 at...

Electron localization in water clusters. II. Surface and internal states

Electron attachment and localization in small water clusters (H2O)n (n=8–128) is studied using path‐integral molecular dynamics simulations. The electron‐water molecule interaction is described via a pseudopotential which includes Coulomb, polarization, exclusion and exchange contributions. Different electron localization modes are found depending on cluster size. For small and intermediate size clusters (n=8–32), the energetically favored localization mode involves a surface state and the calculated excess electron binding energies are in agreement with experimentally measured values. In larger clusters, n=64, 128, internal localization (solvation) is energetically favored. In both cases the localization of the excess electron is accompanied by large cluster molecular reorganization. The cluster size dependence of the localization mode, the energetics, structure, and excess electron distributions in the negative molecular anions (H2O)−n, and the dependence on temperature are explored.

Electron localization in water clusters. I. Electron–water pseudopotential

A local pseudopotential for the interaction of an electron with a water molecule in the electronic ground state is developed. The potential contains Coulomb, adiabatic polarization, exclusion, and exchange contributions. The potential is suitable for a description of excess electron states in water clusers, and for studies of electron solvation in water. Quantum path integral molecular dynamics simulations of electron localization in water clusters employing this potential yield results in agreement with available experimental data and all‐electron quantum chemical calculations.

Size dependence of the energetics of electron attachment to large water clusters

Abstract The size dependence of the binding energy of a localized excess electron in large water clusters originates from long-range polarization interactions. The vertical and adiabatic binding energies of compact, interior excess electron states in (H 2 O) n − clusters, obtained from quantum path-integral molecular-dynamics simulations, exhibit a linear dependence on n − 1 3 , in quantitative agreement with the implications of dielectric medium effects in finite systems.

New equations of motion for molecular dynamics systems that change shape

In the methods of molecular dynamics a large system is frequently subdivided into smaller regions which are periodic replications of each other. This subdivision defines the ‘‘computational cell’’ and may be performed in a number of equivalent ways, all producing cells of different shapes but of the same size and containing equivalent sets of particles. We consider the requirement that the form of the equations of motion be invariant under transformations connecting such equivalent computational cells. Since none of the equations of motion of the Parrinello–Rahman family proposed so far satisfy this condition, we construct some which do. These new equations are more practically useful than their antecedents for studies of systems whose computational cells may undergo extensive changes in shape, especially for studies of yield and flow under an applied stress.

Excess electrons in ammonia clusters

Abstract Electron attachment and localization to ammonia clusters, (NH3)n− (n = 16–256), is studied using path-integral molecular simulations, employing an electron-molecule pseudopotential which includes Coulomb, polarization, exclusion and exchange contributions. Due to the nature of the interactions the ammonia clusters do not form stable well-bound surface states for any size, in contrast to medium-size water clusters. The onset of bound excess electron states in ammonia clusters occurs for n ⩾ 32, via an internal localization mode, in agreement with experiment.

Excess electrons in polar molecular clusters

Electron attachment and localization to water and ammonia clusters, (NH3)−n and (H2O)−n (n=16–256), is studied using path‐integral molecular dynamics simulations, employing an electron–molecule pseudopotential which includes Coulomb, polarization, exclusion, and exchange contributions. Due to the nature of the interactions the ammonia clusters do not form stable well‐bound surface states for any size, in contrast to the case of medium size water clusters. The onset of bound excess electron states in ammonia clusters occurs for size n≥32, via an internal localization mode, in agreement with experimental data.

Structures and spectra of gold nanoclusters and quantum dot molecules

Abstract.Size-evolutions of structural and spectral properties in two types of finite systems are discussed. First we focus on energetics and structures of gold clusters, particularly AuN in the 40≲N≲200 range exhibiting a discrete sequence of optimal clusters with a decahedral structural motiff, and on the electronic structure of bare and methyl-thiol passivated Au38 clusters. Subsequently, bonding and spectra of quantum dot molecules (QDM’s) are investigated, using a single-particle two-center oscillator model and the local-spin-density (LSD) method, for a broad range of interdot distances and coupling strengths. A molecular orbital classification of the QDM states correlates between the united-dot and separated-dots limits. LSD addition energies and spin polarization patterns for QDM’s in the entire coupling range are analyzed, guiding the construction of a constant interaction model. A generalization of the non-interacting-electrons Darwin–Fock model to QDM’s is presented. Wigner crystallization of the electrons leading to formation of Wigner supermolecules is explored in both the field-free case and with a magnetic field using a spin-and-space unrestricted Hartree–Fock method.

Reactions in clusters

A new class of cluster‐catalyzed reactions is proposed and investigated using extensive molecular‐dynamics simulations. These reactions involve the collision between reactants which are embedded in inert gas clusters, whose role is that of a local heat bath (i.e., extended third body). The concept is demonstrated for the reactions [Na4Cl3]+Arq+Cl− (for q=12 and 32) and [Na14Cl12]+2Ar30+Cl− for several initial temperatures, relative translational energies between the reactants, and impact parameters. For associative reactions, i.e., Cl− attachment accompanied by the release of Ar atoms, the dynamics of the reactions, the dependencies of the isomeric structures of the product alkali‐halide clusters, and of reaction pathways and branching ratios on reactant size and reaction conditions are systematically studied.

SMALL IS DIFFERENT

Investigations of variations of physical and chemical properties of clusters as a function of size allow systematic explorations of evolutionary patterns of materials properties from the molecular scale to the condensed phase regimes. Using classical and quantum molecular dynamics simulations we discuss the dynamics of cluster collisions with surfaces leading to shock formation, dielectrons and sodium solvation in water clusters, size-evolutionary patterns of metallization of sodium-fluoride clusters, and the energetics of metal alloy clusters.