The bonding of hydrogen in the Al13H aggregate is analyzed in the framework of density functional theory using the local density approximation. The interaction between the H-1s orbital and only certain molecular orbitals of Al13 is responsible for the binding. Different measures of the charge transfer give consistent results and predict the stabilization of a sizable amount of electronic charge, about two electrons, around the proton site. The state of the H atom can be described as a negatively charged impurity screened by the surrounding electron gas, similarly to a H impurity embedded in a vacancy in metallic aluminum. Friedel-type oscillations can be appreciated in the screening charge. Local Fukui functions and condensed Fukui indexes associated to the ground state of the cluster Al13 are used as indicators of molecular reactivity. Those indices allow to predict and understand the equilibrium location of H found in the total energy calculations for Al13H.

Density-functional calculations predict half-metallicity in zigzag single-walled carbon nanotubes of finite length with the two ends saturated with hydrogen. We have analyzed the change of the ␣-and ␤-spin electronic gaps under the influence of an electric field applied along the nanotube axis. The half-metallic behavior, in which the electronic gap is zero for one spin flavor and nonzero for the other, is obtained for a critical electric field of 3.0/ w V / Å, where w is the length of the nanotube. This critical field is the same as that predicted for graphene nanoribbons. By a detailed analysis of the spin structure of the ground state, we show the relevance of the edge states, electronic states spatially localized at the carbon atoms of the nanotube boundaries, on the on-set of half-metallicity, and on the magnetic properties of the finite semiconducting zigzag nanotubes.

The electronic and geometrical structures of neutral and charged clusters Al N , N < 22, are calculated using ab initio density functional theory (DFT). The geometries obtained are in agreement with those of ab initio molecular dynamics. The binding energy is proportional to the inverse of the cluster radius, with a slight overbinding with respect to the experiment. The vertical ionization potentials follow the oscillations found experimentally. The photoelectron density of states for the anions are in agreement with the experimental results. The s, p, and d characters of the Kohn-Sham orbitals are obtained as a function of the size, and it is shown that the s-p hybridization starts at Al 8 both for neutral and anionic clusters. The deeper Kohn-Sham eigenvalues follow the pattern of a spherical jellium-like model. The occupation of the p band is found to be one electron per atom in the cluster; this agrees with experimental results.PACS. 36.40.Mr Spectroscopy and geometrical structure of clusters -61.46.+w Clusters, nanoparticles

Clusters with closed electronic shells are highly stable, and this is a robust property with far-reaching implications for cluster abundance, cluster shapes, dynamical response to electric fields and cluster dissociation. Clusters with closed shells are also good candidates for the synthesis of novel materials. The density functional formalism is currently used to investigate the self-assembling of metallic clusters to yield cluster solids. A high intrinsic stability is not the only requirement for expecting a successful assembling, and we investigate additional conditions by performing computer simulations for selected examples. The study of the assembling of hydrogen-doped icosahedral aluminium clusters shows that an optimized relative orientation of each cluster unit with respect to all its neighbour clusters becomes a favourable condition. The analysis of the clustering effects observed in crystalline alloys formed by Pb and alkali metals (tetrahedral Pb4 clusters surrounded by the alkali cations form in those alloys) provides further insight: coating passivates the clusters and helps to stabilize the assembled material.

Density functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al 13 H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb 4 clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids.