No abstract

No abstract

No abstract

The periodic ab initio Hartree-Fock approach is applied to the Li, Na, K, Be, Mg, Ca and Mn oxides, and to Al 2 O 3 (corundum) and SiO 2 (a-quartz). A local basis set (‘atomic orbitals’) is used. The equilibrium geometry, the formation energy and the bulk modulus are calculated, with reasonable agreement with experiment. The influence of the environment on the oxygen ions is discussed through the Mulliken population and band structure data.

An ab initio periodic Hartreee-Fock study of sillco-chabalik is presented. Complete geometq optimization has been performed using both a minimal STO-3G and an extended (split-valence plus polarization functions) basis set; the results are similar to those obtained with a semi-classical approach based on a model potential. The more sophisticated basis set has been adopted for characterizing the gmund-state wave-function. The electrostatic potential within the main cavity is calculated and analyzed.

Be and Ca substitutional impurities in bulk MgO are srudied using the periodic ab inirio Hartre?-Fock method and the considerably, simpler ionic model based on two-body forces and the dipole shell model. In both cases a supercell approach was used; in the Harkee-Fock calculations the Izgest unit cell conkCned 32 atoms whilst in the case of the ionic model supercells of up to 250 atoms were employed. The HartreeFock results for the Be and Ca defect f o d o n em-are -4.10 and 6.25 eV respectively, the COnEspondiog results Itom the ionic model being -3.88 and 6.37 eV, these data being defined with respect to ionic, raIher than atomic. substitutions. The effect of such defects an the Hmee-Fock charge distribution of the host is examined in terms of nuclear displacements (these being compared with the corresponding results of the ionic model) 'and eleckonic redistribution, the latter being analysed in terms of c-e density maps and induced atomic multipolcs. The convergeace pattern of the Harlree-Fock deF8ct Formation energies is discussed using data h m cells containing 8.16 and 32 atoms. The larger unit cells employed with the ionic model allow a more careful study of the convergence rate of the superceil method.