No abstract

The electronic charge density of cubic boron nitride is calculated within the ab initio Hartree-Fock approximation using the program CRYSTAL. Based on Debye hypothesis, the thermal motion of atoms is considered by disturbing the atomic orbitals by mean-square displacements given from experiment. The calculated difference charge density obtained by subtraction of the total density and that of an independent atomic model (IAM) is characterized by a charge-density accumulation between next neighbours slightly shifted towards the nitrogen. The calculated X-ray structure amplitudes are compared with two different data sets [Josten (1985). Thesis.

The electronic structure of lithium sulfide and sodium sulfide in the solid state is established at the Hartree-Fock level, as implemented in the CRYSTAL program. Two all-electron basis sets are adopted for each compound. Mulliken analysis, band structure, density of states and electron charge density are studied: these exhibit the highly ionic character of both compounds; this seems rather surprising because of the weak electronegativity and large polarizability of the sulfur. Form factors of the sulfur are deduced from the structure factors and analysed in order to show the deformations of this anion in the crystal environment. Finally, electron momentum density and Compton profiles confirm the almost fully ionic character of the compounds and prove the quality of the calculated wavefunctions.

No abstract

Carbon substitution in bulk silicon has been investigated using the super-cell approach, in conjunction with the periodic ab initio Hartree-Fock method. The convergence of the defect formation energy and of the relaxed defect geometry as a function of the super-cell size is discussed with reference to super-cells containing 8, 16, 32 and 64 atoms. It turns out that the convergence of the unrelaxed defect formation energy is rapid, in spite of the large local charge redistribution around the defect (the net charge on carbon is 1.2 mod e mod ); the relaxation effects are very large (about 2.0 eV) and involve mainly the first and second neighbours; however, the relaxation of the fifth neighbours of the defect (which is possible only with the biggest super-cell considered) lowers the energy by a further 0.06 eV. The defect formation energy and the atomic displacements obtained with the 32 and 64 atoms super-cells are similar, whereas the energy difference between the 16 and 32 atoms cells is as large as 0.4 eV.