The Young modulus for crystalline polyethylene is calculated using ab initio molecular dynamics based on density functional theory in the local density approximation (DFT-LDA). This modulus, which can be seen as the ultimate value for the Young modulus of polyethylene fibers, is found to be 334 GPa. For the first time the modulus is evaluated ab initio (no bias from experimental data) with demonstrated basis set convergence.

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We performed density-functional-theory total-energy and force calculations of clean and Na-covered GaAs(110) for two coverages: e= -, ' (one Na atom per pair of Ga and As) and e= -, '. The atomic positions of the adsorbate and of the top three substrate layers are fully relaxed. We discuss the nature of the adsorbate-substrate interaction, the adsorbate-adsorbate interaction, how the adsorbate modifies the clean-surface geometry, and how the atomic and electronic structures are correlated. It is found that the adsorption is partially ionic. The highest occupied adsorbate-induced "band" refIects the properties of a Mott-insulator state, whereas the lowest unoccupied state is a one-dimensional Bloch state. PACS numbers: 73.20.At, 68.35.Fx, 73.30.+yThe electronic, optical, magnetic, and chemical properties of polyatomic systems are sensitively affected by the atomic geometry. In this respect semiconductor surfaces have attracted particular attention in recent years, the most studied system being GaAs (110). It is now well established that the GaAs(110) surface atoms relax by a significant amount such that the As anions move outwards and the Ga cations move inwards (e.g. , Ref.[1] and references therein) (see Fig. 1).There is an ongoing debate as to how alkali atoms adsorb on this surface, and how they modify the surface geometry. The plausible idea of an ionic adsorption is often questioned [2], because the Coulombic repulsion should hinder the experimentally observed close packing at low coverage. With respect to the surface geometry it is usually assumed that even a low adsorbate coverage will unrelax the surface completely [2-5], but a proof of this assumption is lacking. These aspects are crucial for a better understanding of Schottky-barrier formation and the nature of the electronic states responsible for the Fermi-level pinning (see, for example, Ref.[6] and references therein).In this paper we present parameter-free calculations addressing the above-mentioned questions. We will show that the alkali adsorbate is partially ionized, and that it can move rather freely with low migration barriers over the surface. However, the diffusion should be practically one dimensional.In contrast to the generally applied assumption we find that at low coverage the clean-surface relaxation is significantly modified, but it is not removed. Furthermore, the standard argument that the (partial) ionization of the alkali adsorbate should induce a strong adsorbate-adsorbate repulsion is shown to be incorrect.We find a very localized polarization of the surface electron density.This implies an e%cient screening, by which, together with the coverage dependence of the lattice relaxation, the direct ion-ion repulsion term is more than compensated. Of particular interest are the highest occupied and the lowest unoccupied adsorbate-induced states, as they have qualitatively different character. Whereas the highest occupied Na-induced state is best understood in a localized picture as a Mott-insulator state, the lowest unoccupied state h...

Self-consistent calculations of the electronic structure of P-PbOz, using the augmented-spherical-wave method and the ab initio pseudopotential method, are presented. The band structure consists of a set of nonbonding oxygen 2p bands and a very broad band, formed by strong hybridization of Pb 6s and O 2p orbitals. The calculations indicate that P-Pb0z is a semimetal, with holes of 0 2p character and electrons of mixed Pb 6s -0 2p character. The relation with the electronic structure and the properties of BaPbl "Bi"O3 is discussed.

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