Band structure calculations oxide materials

A number of LDA band structure calculations by other groups were performed in order to predict the electronic spin state for parent materials of Cu oxide superconductors. The oxides were found to be metallic and nonmagnetic, unlike the experimental evidence of insulating and antiferromagnetics. 

In the pseudopotential method, core states are omitted from explicit consideration, a plane-wave basis is used, and no shape approximations are made to the potentials. This method works well for complex solids of arbitrary structure (i.e., not necessarily close-packed) so long as an adequate division exists between localized core states and delocalized valence states and the properties to be studied do not depend upon the details of the core electron densities. For materials such as ZnO, and presumably other transition-metal oxides, the 3d orbitals are difficult to accommodate since they are neither completely localized nor delocalized. For example, Chelikowsky (1977) obtained accurate results for the O 2s and O 2p part of the ZnO band structure but treated the Zn 3d orbitals as a core, thus ignoring the Zn 3d participation at the top of the valence region found in MS-SCF-Aa cluster calculations (Tossell, 1977) and, subsequently, in energy-dependent photoemission experiments (Disziulis et al., 1988). 

Many transition-metal oxides and fluorides are insulators which LSD incorrectly describes as metals[119]. For some of these materials, full-potential GGA calculations open up a small fundamental gap which LSD misses, and so correct the description. For others, GGA enhances the gap, and generally improves the energy bands[l 19]. Of course, the sizes of the band-structure gaps are not physically meaningful in LSD, GGA, or even with the exact Kohn-Sham potential for the neutral solid. To get a physically meaningful fundamental gap, one must take account... 

Also in their recent work on crystalline ZnO, Massidda et a/.89 found that the band structures were too wide with Hartree-Fock calculations, but here the LDA bands were somewhat too narrow. ZnO may actually be a material for which correlation effects are quite important (as also is the case for the high- Tc superconductors, other transition-metal oxides, some heavy-fermion systems,... 

One of the early triumphs of quantum mechanics in the area of solid state materials was the ability to explain why certain materials are metallic conductors while others are insulating or semiconducting. The energy band structure, which can be calculated by ab initio or semiempirical methods, provides access to these electrical properties. As will be shown below, DFT methods are well suited to treat metallic systems whereas there are problems in the accurate prediction of energy band gaps in semiconductors and insulators. Furthermore, certain transition metal oxides and solids containing rare-earth and actinide elements present serious theoretical challenges which have not been completely resolved yet,... 

---