Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminum clusters containing up to 15 atoms. The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, fragmentation channels, ionization potential, and vertical and adiabatic electron affinities as a function of size. The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster contains 6 atoms. An interior atom emerges only when clusters contain 11 or more atoms. The geometrical changes are accompanied by corresponding changes in the coordination number and the electronic structure. The latter is reflected in the relative concentration of the s and p electrons of the highest occupied molecular orbital. Aluminum behaves as a monovalent atom in clusters containing less than seven atoms and as a trivalent atom in clusters containing seven or more atoms. The binding energy evolves monotonically with size, but Al7, Al7+, Al7−, Al11−, and Al13− exhibit greater stability than their neighbors. Although the neutral clusters do not conform to the jellium model, the enhanced stability of these charged clusters is demonstrated to be due to the electronic shell closure. The fragmentation proceeds preferably by the ejection of a single atom irrespective of the charge state of the parent clusters. While odd-atom clusters carry a magnetic moment of 1μB as expected, clusters containing even number of atoms carry 2μB for n⩽10 and 0 μB for n>10. The calculated results agree very well with all available experimental data on magnetic properties, ionization potentials, electron affinities, and fragmentation channels. The existence of isomers of Al13 cluster provides a unique perspective on the anomaly in the intensity distribution of the mass spectra. The unusual stability of Al7 in neutral, cationic, and anionic form compared to its neighboring clusters is argued to be due to its likely existence in a mixed-valence state.

A comprehensive theoretical study of the geometries, energetics, and electronic structure of neutral and charged 3d transition metal atoms (M) interacting with benzene molecules (Bz) is carried out using density functional theory and generalized gradient approximation for the exchange-correlation potential. The variation of the metal-benzene distances, dissociation energies, ionization potentials, electron affinities, and spin multiplicities across the 3d series in MBz complexes differs qualitatively from those in M(Bz)(2). For example, the stability of Cr(Bz)(2) is enhanced over that of CrBz by almost a factor of 30. On the other hand, the magnetic moment of Cr(Bz)(2) is completely quenched although CrBz has the highest magnetic moment, namely 6 mu(B), in the 3d metal-benzene series. In multidecker complexes involving V(2)(Bz)(3) and Fe(2)(Bz)(3), the metal atoms are found to couple antiferromagnetically. In addition, their dissociation energies and ionization potentials are reduced from those in corresponding M(Bz)(2) complexes. All of these results agree well with available experimental data and demonstrate the important role the organic support can play on the properties of metal atoms/clusters.

The electronic and geometrical structures of the ground and excited states of the 3d-metal dioxides ScO2, TiO2, VO2, CrO2, MnO2, FeO2, CoO2, NiO2, CuO2, and ZnO2 along with their singly charged negative ions have been calculated using the density functional theory with generalized gradient approximation for the exchange-correlation potential. We have considered oxo, peroxo, and superoxo isomers in both neutral and anionic series. The ground states of all 3d-metal dioxides and their anions possess oxo forms, except for copper dioxide, which prefers a superoxo form. All the dioxides have a large number of isomers closely spaced in total energy. The champion, FeO2 -, possesses five isomers within an energy range of 0.35 eV. The energy gap between oxo, peroxo, and superoxo isomers decreases along the series from TiO2 to ZnO2. ScO2 has peroxo and superoxo isomers which are close in total energy to its oxo ground state. The electron affinities are computed for all types of isomers and are compared to available experimental data. On the average, our theoretical values are within 0.2 eV from the corresponding experimental data. All the ground-state dioxides and their anions are found to be thermodynamically stable except for ZnO2, which is unstable toward dissociation of molecular oxygen.

First-principles calculations based on the generalized gradient approximation to the density functional theory are performed to explore the global geometries, ground-state spin multiplicities, relative stabilities, and energetics of neutral and anionic V(n)(Bz)(m) (n=1-3, m=1-4, with n

The electronic and geometrical structure of the ground and excited states of the 3d metal monoxide anions ScO-, TiO-, VO-, CrO-, MnO-, FeO-, CoO-, NiO-, CuO-, and ZnO- were calculated using density functional theory and different formulations of generalized-gradient approximations for the exchange-correlation potential. It was found that the anion states with low- and high-spin multiplicities with respect to the ground-state spin multiplicities of the corresponding neutral parents are stable toward autodetachment of the extra electron. All the low-spin multiplicity anion states are more stable than the high-spin ones, except for that of CrO-, whose ground state appears to be a high-spin multiplicity state. Computed electron affinities of the neutral monoxides are in good agreement with the experimental values obtained by laser photoelectron spectroscopy. It is shown that pure density functional methods are generally superior to a hybrid Hartree−Fock density-functional-theory approach, except for reproducing bond rupture energies.

Using self-consistent-field molecular-orbital theory, we show that the interaction of hydrogen molecules with a Ni + ion is characteristically different from that with a neutral Ni atom. While hydrogen chemisorbs dissociatively on the neutral metal atom, it is bound to the cation in its molecular form. The atomic bonding is a consequence of the Pauli exclusion principle whereas the bonding of the molecular hydrogen results from an electrostatic interaction. We predict that a Ni + ion can bind at least six hydrogen molecules. PACS numbers: 3l.20.Ej, 35.20.Gs The interaction of hydrogen with metals and metal surfaces has been studied for many years. It is commonly understood that a H2 molecule breaks up into individual atoms at about 0.5 A from the metal surface and atomic chemisorption ensues [1]. No evidence exists, to our knowledge, of molecular chemisorption of hydrogen on the metal surface or inside the bulk metal. However, pairing of hydrogen mediated by a metal atom in rareearth systems [2] and molecular hydrogen in Si [3] have recently been observed.Little is known about the interaction of hydrogen with small metal particles consisting of a few atoms. The first evidence that hydrogen interaction with metal clusters may be fundamentally different from bulk came from recent experiments [4] on hydrogen reactivity and absorption. The reactivity was found to change by orders of magnitude by changing only a few atoms in the cluster. Equally interesting was the observation [5] that metal clusters might be able to absorb as many as eight hydrogen atoms per metal atom even though no corresponding bulk metal hydride exists. Not much is known regarding why clusters behave so differently towards hydrogen than bulk or whether hydrogen is bonded to the cluster in atomic or in molecular form.In this Letter we show that hydrogen interacts the same way with a neutral atom as it does with the atoms on surfaces and in the bulk. However, the interaction of hydrogen with a metal ion is fundamentally different. In the case of the neutral atom, an electron is transferred from the metal atom to the approaching H2 molecule and, in keeping with the Pauli exclusion principle, occupies the antibonding orbital of the molecule. This, in turn, breaks the molecular bond in favor of atomic bonding between the dissociated hydrogen atoms and the metal atom. However, when a H2 molecule approaches a transition-metal ion, it becomes energetically inefficient for the metal atom to donate an electron to H2 since the second ionization potential of the metal atom is rather high. Instead, the ion polarizes the H2 molecule and the bonding between the ion and the H2 molecule is governed by a dipole mechanism. We predict that a single Ni + ion can trap up to ten hydrogen molecules.Our results are based on first-principles calculations of

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.

No abstract