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We have investigated various electronic band structures for different graphite modifications using the extended tight binding method. The specific crystal structures studied are the two naturally occurring graphite modifications (Bernal and rhombohedral), and a hypothetical configuration where the carbon atoms in consecutive layers are directly above each other. The latter structure has not been observed in pure graphite, but it is the backbone structure for different stages of intercalated graphite, e.g. Li-graphite compounds LiC n . On comparing band structures for various graphite modifications we find important differences in the π-bands close to the Fermi energy, the region dominating transport and low energy excitation properties.

The regular adsorption of CO on a MgO(001) monolayer has been investigated at an “ab initio Hartree-Fock crystalline-orbital level, with a basis set containing 9 atomic orbitals per atom. Both vertical bindings to Mg (through oxygen and carbon) have been considered and the C-O as well as the Mg-CO or Mg-OC distances have been optimized. At the 1/2 surface coverage (one CO molecule every two surface Mg ions) adsorption energies of 4.13 (carbon binding) and 3.29 (oxygen binding) kcal/mol were obtained, to be compared with an experimental adsorption heat of 3.6 kcal/mol. Similar energies were obtained for the 1/4 coverage, whereas the 1/1 adsorption is less favoured, due to the repulsion between the CO molecules. The charge redistributions in the CO molecule and in the substrate are considered, as well as the changes occurring in the density of states for the interacting systems.

The layered mineral brucite Mg(OH${)}_{2}$ is investigated theoretically using an ab initio all-electron linear combination of atomic orbitals Hartree-Fock (HF) approximation. At the HF level, the interlayer interaction is weak and the interlayer distance is larger than the experimental one. Bonding is discussed on the basis of density of states and charge-density maps. No hydrogen bond is characterized. A posteriori correction of the energy for the correlation error is performed by use of the functional approach. The three semilocal functional formulas used yield similar results. This brings in extra interlayer bonding interaction, and yields a calculated geometry in agreement with experiments. The analysis of the interlayer bondings shows that it is mainly of dispersion type, and that the used functionals account for dispersion, in particular at short interatomic distances.

Three different crystal phases of magnesium oxide [B1 (NaCl), B2 (CsCl), and B${8}_{1}$ (inverse NiAs)] are investigated theoretically. An ab initio all-electron linear combination of atomic orbitals Hartree-Fock approximation is adopted; extended basis sets are used which have been variationally optimized in each case. In ordinary conditions, the B${8}_{1}$ and B2 structures result less stable with respect to the rocksalt structure by 0.44 and 1.77 eV, respectively. With increasing pressure, the transition B1\ensuremath{\rightarrow}B2 is estimated to occur around 2.2 Mbar, while it is not excluded that a pressure interval exists around 2 Mbar where the B${8}_{1}$ phase is the most stable of the three structures. Data on the electronic properties of the three phases are provided and discussed: Mulliken populations, charge-density distribution, electron momentum distribution and anisotropy, magnesium core deformation, and band structure.