An ab initio Hartree-Fock study of alpha-MoO3

The structural and ground-state electronic properties of the layered orthorhombic molybdenum trioxide (alpha-MoO3) have been investigated using a periodic boundary condition ab initio Hartree-Fock method. The effect of electron correlation is estimated with a posteriori density functional corrections to the total energy. The structure has been optimised using the correlation corrected Hartree-Fock energy (except for the interlayer separation where the correlation correction has not been applied), treating each degree of freedom independently. The bonding in the solid is examined in detail: the nature of the Mo-O interaction 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 of covalence in the bonding is characterised using electron density maps and the Mulliken population analysis, the net charge shows a value as low as -0.44\e\ for the oxygen involved in the shortest Mo-O bond. The energy as a function of the interlayer spacing has a minimum, both at the correlated and the Hartree-Fock level, indicating that a weak attractive Coulombic force is active between adjacent layers.