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The static and dynamic structure of liquid Al is studied using the orbital free ab initio molecular dynamics method. Two thermodynamic states along the coexistence line are considered, namely Tϭ943 and 1323 K, for which x-ray and neutron scattering data are available. A kinetic-energy functional which fulfills a number of physically relevant conditions is employed, along with a local first-principles pseudopotential. In addition to a comparison with experiment, we also compare our ab initio results with those obtained from conventional molecular-dynamics simulations using effective interionic pair potentials derived from second-order pseudopotential perturbation theory.

The structural and bonding properties of small neutral alkali halide clusters, (AX) n with nр10, A ϭLi ϩ ,Na ϩ ,K ϩ ,Rb ϩ , and XϭFϪ , are studied using the ab initio perturbed ion ͑PI͒ model and a restricted structural relaxation criterion. A trend of competition between rocksalt and hexagonal ringlike isomers is found and discussed in terms of the relative ionic sizes. The main conclusion is that an approximate value of r C /r A ϭ0.5 ͑where r C and r A are the cationic and anionic radii͒ separates the hexagonal from the rocksalt structures. The classical electrostatic part of the total energy at the equilibrium geometry is enough to explain these trends. The magic numbers in the size range studied are nϭ4, 6, and 9, and these are universal since they occur for all alkali halides and do not depend on the specific ground-state geometry. Instead those numbers allow for the formation of compact clusters. Full geometrical relaxations are considered for (LiF) n (nϭ3 -7) and (AX) 3 clusters, and the effect of Coulomb correlation is studied in a few selected cases. These two effects preserve the general conclusions achieved thus far. ͓S0163-1829͑97͒01848-1͔

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Putative global minima of neutral ͑Al n ͒ and singly charged ͑Al n + and Al n − ͒ aluminum clusters with n = 13-34 have been located from first-principles density functional theory structural optimizations. The calculations include spin polarization and employ the generalized gradient approximation of Perdew, Burke, and Ernzerhof to describe exchange-correlation electronic effects. Our results show that icosahedral growth dominates the structures of aluminum clusters for n = 13-22. For n = 23-34, there is a strong competition between decahedral structures, relaxed fragments of a fcc crystalline lattice ͑some of them including stacking faults͒, and hexagonal prismatic structures. For such small cluster sizes, there is no evidence yet for a clear establishment of the fcc atomic packing prevalent in bulk aluminum. The global minimum structure for a given number of atoms depends significantly on the cluster charge for most cluster sizes. An explicit comparison is made with previous theoretical results in the range n = 13-30: for n = 19, 22, 24, 25, 26, 29, 30 we locate a lower energy structure than previously reported. Sizes n = 32, 33 are studied here for the first time by an ab initio technique.