AN AB-INITIO HARTREE-FOCK STUDY OF ALPHA-MOO3

The structural and ground-state electronic properties of the layered o rthorhombic molybdenum trioxide (alpha-MoO3) have been investigated us ing a periodic boundary condition ab initio Hartree-Fock method. The e ffect of electron correlation is estimated with a posteriori density f unctional corrections to the total energy. The structure has been opti mised using the correlation corrected Hartree-Fock energy (except for the interlayer separation where the correlation correction has not bee n applied), treating each degree of freedom independently. The bonding in the solid is examined in detail: the nature of the Mo-O interactio n changes considerably with the equilibrium bond distance, and varies from strongly covalent for the shortest bond to a predominantly ionic interaction for the longest bonds in the MoO6 octahedron. The degree o f covalence in the bonding is characterised using electron density map s and the Mulliken population analysis, the net charge shows a value a s low as -0.44\e\ for the oxygen involved in the shortest Mo-O bond. T he energy as a function of the interlayer spacing has a minimum, both at the correlated and the Hartree-Fock level, indicating that a weak a ttractive Coulombic force is active between adjacent layers.