Using molecular orbital approach and the generalized gradient approximation in the density functional theory, we have calculated the equilibrium geometries, binding energies, ionization potentials, and vertical and adiabatic electron affinities of Si n O m clusters (nр6,mр12). The calculations were carried out using both Gaussian and numerical form for the atomic basis functions. Both procedures yield very similar results. The bonding in Si n O m clusters is characterized by a significant charge transfer between the Si and O atoms and is stronger than in conventional semiconductor clusters. The bond distances are much less sensitive to cluster size than seen for metallic clusters. Similarly, calculated energy gaps between the highest occupied and lowest unoccupied molecular orbital ͑HOMO-LUMO͒ of (SiO 2) n clusters increase with size while the reverse is the norm in most clusters. The HOMO-LUMO gap decreases as the oxygen content of a Si n O m cluster is lowered eventually approaching the visible range. The photoluminescence and strong size dependence of optical properties of small silica clusters could thus be attributed to oxygen defects.

Using a combination of classical molecular dynamics simulation and first principles molecular orbital theory, we provide the first comprehensive study of the equilibrium geometries, energetics, electronic structure, vertical ionization potential, and magnetic properties of Ni clusters containing up to 21 atoms. The molecular dynamics simulation makes use of a tight binding many-body potential, while the calculations based on molecular orbital theory are carried out self-consistently using the numerical atomic bases and the density functional theory. The adequacy of the molecular dynamics results on the energetics and equilibrium geometries is tested by comparing the results with those obtained from the self-consistent molecular orbital theory for clusters of up to six atoms. For larger clusters, equilibrium geometries were obtained from molecular dynamics simulation, and their electronic structure and properties were calculated using molecular orbital theory without further geometry reoptimization. Frozen core and local spin density approximations were used in the molecular orbital calculations. In small clusters (n ≤ 6), the calculations were repeated by including all electrons and the gradient correction to the exchange−correlation potential. The calculated vertical ionization potential and magnetic moments of Ni clusters are compared with recent experimental data.

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