B. K. Rao

Structural and electronic properties of compound metal clusters

The equilibrium geometries, relative stabilities, and vertical ionization potentials of compound clusters involving Lin, Na, Mg, and Al atoms have been calculated using ab initio self-consistent field linear combination of atomic orbitals — molecular orbital (SCF-LCAO-MO) method. The exchange energies are calculated exactly using the unrestricted Hartree-Fock (UHF) method whereas the correlation correction is included within the framework of configuration interaction involving pair excitations of valence electrons. While the later correction has no significant effect on the equilibrium geometries of clusters, it is essential for the understanding of relative stabilities. Clusters with even numbers of electrons are found to be more stable than those with odd numbers of electrons regardless of their charge state and atomic composition. The equilibrium geometries of homo-nuclear clusters can be significantly altered by replacing one of its constituent atoms with a hetero-nuclear atom. The role of electronic structure on the geometries and stabilities of compound clusters is discussed.

Clusters—A new phase of matter

Abstract Atomic clusters formed by agglomeration of atoms constitute a new class of matter with structural and electronic properties that depend uniquely on their size. Through innovative experiments and detailed theoretical calculations, it is now possible to understand how the various properties of a solid evolve as constituent atoms come together. The studies shed new light on how bonds in molecules transform to bands in solids, insulators become metals, and reactivity turns into passivity. The strong and unusual dependence of electronic properties on cluster size and geometry is providing ideas for novel technology that may lead to a new era of atomic engineering. Some of the recent developments in the field of clusters are reviewed with particular emphasis on cluster properties that are quite distinct from their atomic or solid state behavior.

Modelling of metal clusters as fragments of crystalline solids

The structural and electronic properties of pure and hydrogenated Li metal have been calculated by modelling the crystalline solid in terms of small clusters of constituent atoms. Using the self-consistent molecular orbital method in the Hartree-Fock approximation, the authors have investigated the minimum number of atoms necessary to distinguish among various crystalline phases. They find that with as few as three atoms in the molecular cluster, the BCC phase can be shown to be the most stable structure. They have calculated the dependence of the optimised lattice parameter as a function of cluster size for both FCC and BCC phases of Li. While the calculated lattice constants converge to the bulk value with only about 20 atoms in the cluster, the binding energy per atom shows no sign of saturation. Thus how well the properties of molecular clusters represent bulk characteristics depends on the properties being investigated. This aspect is further illustrated by calculating the equilibrium site of hydrogen in Li. It is shown that for small clusters consisting of less than 20 atoms, the preferential site of hydrogen depends not only on the cluster size, but also on the way the cluster is constructed. The distortion of the nearest neighbour atoms around the hydrogen atom is studied by minimising the total energy with respect to the nearest-neighbour Li-hydrogen distance. Contrary to the conventional wisdom, they find that the metal atoms surrounding hydrogen are not always displaced outward. For example, the Li atoms around an octahedral hydrogen atom relax outward, while those around a tetrahedral hydrogen atom relax inward. In both cases, the equilibrium distance between hydrogen and nearest Li atom is 3.48 au. This is comparable with the corresponding distance of 3.6 au in stoichiometric lithium hydride. They have also calculated the charge transfer between Li and hydrogen atoms and the population of various orbital states for all the Li atoms. The results are used to analyse the screening of impurities in the metallic environment and to shed light on Mossbauer isomer shifts in metal-hydrogen systems.

Geometries and electronic structures of negatively charged carbon clusters

Self-consistent field molecular orbital calculations have been performed on neutral and negatively charged clusters of carbon atoms using an extended basis set designed to obtain correct electron affinity. Correlation effects have been included perturbatively up to second order. The optimized geometries of theCn− (n ≤ 7) anions are all linear chains as observed in experiments. The calculated electron affinities are comparable with experimental data. Studies of the stabilities of doubly charged anions show that clusters uptoC7−− are unstable.

Inter-ring rotational potential of biphenyl and its effect on poly(p-phenylene)

Detailed ab initio studies have been done on the inter-ring torsional states of the biphenyl molecule using self-consistent field molecular orbital method. The potential goes through a minimum at an angle of 38°. The height of the potential barrier for the coplanar state is 2.01 kcal/mol. When the phenyl rings are perpendicular to each other, this height increases to 2.37 kcal/mol. The role of correlation and polarization is found to be important. The shape of the potential suggests that polyparaphenylene may possibly exist as a super helix.

A Hartree-Fock Study of Small Carbon Cluster Anions

Results from Hartree-Fock calculations on optimized C n - clusters are presented for n ≤ 7. An extended basis set has been used for these calculations to obtain correct electron affinities. Equilibrium geometries are all linear. The geometries, binding energies, and electron affinities are discussed in the light of available theoretical and experimental data.

Many-body theory for the antishielding factor of lithium atom

The Sternheimer anti-shielding factor of lithium atom has been calculated using linked cluster many-body perturbation theoretical technique. The results obtained compare well with some of the values available in the literature.

A Fragment Cluster Study of Polyacetylene

Using CH fragments to obtain an optimized cluster, a model calculation has been made for polyacetylene. The model reproduces the bond alternation as expected due to Peierls condition. After removing the errors due to a finite length of the chain, a complete rotational potential energy surface has been generated for the cis-transoidal isomer of polyacetylene. The potential depends upon rotations about two successive single bonds in the backbone. The potential surface indicates the possibility of formation of a super helix.