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The study of defects in metals and ionic solids has been the subject of great theoretical and experimental interest, in basic as well as applied research areas. The present work collects two series of calculations on the energetics of a variety of defective systems, in a metal host matrix (Al), and in two ionic oxides (MgO and Li20) . The energetics and the electronic ground state of the vacancy, of the self-interstitial, and of the hydrogen impurity systems in Al were investigated. The formation and migration energies of Schottky defects in MgO and Frenkel Defects in Li20 were also studied. All results are in close agreement with experiment, while the work gives new insight into the localisation of defects, the role played by lattice relaxation effects, and the defectinduced redistribution of valence electrons. The calculations are based on density functional and pseudopotential theory, make use the supercell approach, and employ in different implementations the conjugate gradients technique to minimise the total energy functional. For the calculations on oxides, we made use of a newly developed parallel computing methodology

The basic properties of the rare earth metals, including single crystal growth, crystal and magnetic structures, and the relationship between electronic and magnetic structure, are reviewed. The problems encountered by the theoretical treatment of the partially occupied, but highly localised, lanthanide 4f levels as bands are discussed, and bandstructure calculations presented for the hexagonal close-packed rare earths. These are compared with available experimental and theoretical data. It is suggested that the exchange-splitting of the lanthanide valence bands may well persist in the paramagnetic state, and that account should be taken of the localised 4f moments in future calculations. The difficulties associated with the preparation of clean single crystal rare earth surfaces are described. The origin of the surface-orderdependent state seen in angle-resolved UV photoemission (ARUPS) spectra from rare earth (0/001) surfaces is discussed. (7 x 1) reconstructions of the (1120) surfaces of Ho, Er and Y are reported, with the resulting surface geometric and electronic structure being indistinguishable from those of the ideal (0001) structure. Momentum-resolved inverse photoernission measurements are presented for Y(000l), with results in good agreement with the calculated bandstructure. A comprehensive ARUPS study of the valence band of Ho(OOOl) is reported, and the results demonstrated to be entirely explicable in terms of emission from one-electron states. ARUPS data from Y(000l), Gd(000l) and Tb(000l) are presented, discussed in the light of the Ho results, and the conclusions of previous ARUPS studies of these surfaces revealed to be in error. Essentially similar ARUPS features are seen on all hcp rare earth (0001) surfaces so far studied and it is suggested that all other such surfaces will show the same features. The Ho(000l) 5p levels are shown to have significant band character, suggesting that further refinements to the band structure calculations are required.

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