Localized orbital description of electronic structures of extended periodic metals, insulators, and confined systems: density functional theory calculations

We present a simple and general method for construction of localized orbitals to describe an electronic structure of extended periodic metals and insulators as well as confined systems. Spatial decay of these orbitals is found to exhibit exponential behavior for insulators and power law for metals. While these orbitals provide a clear description of bonding, they can be also used to determine polarization of insulators. Within density functional theory, we illustrate applications of this method to crystalline aluminium, copper, silicon, PbTiO3, and molecules, such as ethane and diborane

Wannier orbital overlap population (WOOP), Wannier orbital position population (WOPP) and the origin of anomalous dynamical charges

Most d0 transition metal (TM) oxides exhibit anomalously large Born dynamical charges associated with off-centering or motion of atoms along the TM-O chains. To understand their chemical origin, we introduce "Wannier orbital overlap population" (WOOP) and "Wannier orbital position population" (WOPP) in terms of the Wannier function based description of electronic structure obtained within first-principles density functional theory. We apply these concepts in a precise analysis of anomalous dynamical charges in PbTiO3, BaTiO3 and BaZrO3 in the cubic perovskite structure. Determining contributions of different atomic orbitals to the dynamical charge and their break-up into local polarizability, charge transfer and covalency, we find that p orbitals of oxygen perpendicular to the -TM-O- chain contribute most prominently to the anomalous charge, by facilitating a transfer of tiny electronic charge through one unit cell from one TM atom to the next. Our results explain why the corner-shared linkage of TMO6 octahedra, as in the perovskite structure, is ideal for large dynamical charges and hence for ferroelectricity