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LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.

The line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry have been applied for symmetry analysis of double-wall boron nitride and titania nanotubes (DW BN and TiO 2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of hexagonal structure with the same or opposite orientation of translation and chiral vectors. We have considered the two sets of commensurate DW BN and TiO 2 NTs with either armchair-or zigzag-type chiralities, i.e., (n 1 ,n 1 )@(n 2 ,n 2 ) or (n 1 ,0)@(n 2 ,0), respectively. To establish the equilibrium interwall distances corresponding to the minima of energy, we have varied chiral indices n 1 and n 2 of the constituent single-wall (SW) nanotubes. To analyze the structural and electronic properties of hexagonal DW NTs, we have performed ab initio LCAO calculations using the hybrid HartreeÀFock/KohnÀSham exchange-correlation functional PBE0 as implemented in CRYSTAL-09 code. The inversely stacked structure of zigzag-type DW BN NT, characterized by arrangement of positively and negatively charged rings in each atomic cross section (consisting of either B or N atoms, respectively), has been found to be energetically more preferable as compared to the straightly stacked structure containing nanotube rings consisting of the same type of atoms in cross sections, i.e., B(N) and B(N). In armchair-type DW BN NTs, each atomic ring contains the whole number of BÀN bonds, which reduces the electrostatic interaction between both walls. On the other hand, main contribution to interwall bonding in DW TiO 2 NTs is provided by interaction between the nearest oxygen and titanium ions of neighboring shells. The interaction between the walls results in a decrease of band gaps for double-wall NTs as compared to those for SW NTs, which is substantially larger for TiO 2 .

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The results of the first LCAO DFT calculations of cohesive energy, band structure and charge distribution in uranium nitride (UN) crystal are presented and discussed. The calculations are made with the uranium atom relativistic effective core potentials, including 60, 78 and 81 electrons in the core. It is demonstrated that the chemical bonding in UN crystal has a metallic–covalent nature. Three 5f‐electrons are localized on the U atom and occupy the states near the Fermi level. The metallic nature of the crystal is due to the f‐character of both the valence‐band top and the conduction‐band bottom. The covalent bonds are formed by the interaction of 7s‐ and 6d‐states of the uranium atom with the 2p‐states of the nitrogen atom. It is shown that the inclusion of 5f‐electrons in the atomic core introduces small changes in the calculated cohesive energy of UN crystal and electron charge distribution. However, the inclusion of 5s‐, 5p‐, 5d‐electrons in the valence shell allows the better agreement with the calculated and experimental cohesive‐energy value. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)