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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.

First principles calculations are used to investigate carbonated hydroxyapatite, a naturally occurring mineral and also the inorganic component of animal bone. Two types of carbonate substitution are studied:  A-type in which the carbonate ion substitutes for an OH and B-type where the substitution is for a phosphate. Both types have unbalanced charges and various forms of charge compensation are treated. The methods, which are based on density functional theory and first principles pseudopotentials, yield equilibrium atomic arrangements, changes in lattice parameters, and total energies for different types of substitution. When calculated energies of selected stable compounds are used, the formation energies of different carbonate substitutions with accompanying charge compensation defects can be compared. The results indicate that compact complexes are energetically favored, and a B-type material with charge compensation by a calcium vacancy together with a hydrogen atom which bonds to a neighboring phosphate is the most stable of all those considered.

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An ab initio study of four different stoichiometric apatites ͑oxyapatite, hydroxyapatite, fluorapatite, and chlorapatite͒ is presented. The calculations were performed using density-functional theory with the localdensity approximation for exchange and correlation, and a full relaxation of the electronic structure, the atomic arrangement, and the unit cell. Hexagonal unit cells were obtained for all four apatites, and the calculated atomic arrangements are in close agreement with observation in those cases for which the structure is firmly established. A zero-temperature structure is predicted for oxyapatite, and two possible configurations were found for the Cl Ϫ ions in chlorapatite. The possibility of the monoclinic structure in hydroxyapatite and chlorapatite was also studied but no indication of greater stability with respect to the hexagonal structure was found. A relationship between the structure of the apatites and that of pure calcium is discussed.

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Oligoacenes C 4n+2 H 2n+4 ͑n =2, ... ,6͒ are studied using a variety of ab initio methods. Density functional theory ͑DFT͒ optimized geometries were in good agreement with experiment. Vertical and adiabatic ionization potentials and electron affinities were computed with DFT and it was found that standard exchange-correlation ͑xc͒ functionals underestimate ionization potentials in oligoacenes. Possible reasons for this underestimation are discussed. Low lying electronic excitations were computed using time-dependent density functional theory, configuration interaction singles, and configuration interaction singles with approximate treatment of doubles. In agreement with earlier work, time-dependent DFT in conjunction with standard xc-energy functionals substantially underestimates the lowest ͑p͒ singlet-singlet electronic transition.