Embedded cluster model of NbO2: Compton profile and electronic spectra

The energy levels, momentum density, and Compton profile of the rutile phase of NbO2 have been calculated in a self-consistent molecular-cluster model. The cluster is embedded in the crystal lattice and the entire system treated iteratively in the one-electron local-density theory. Nearest-neighbor (NbO6) and second-neighbor (Nb3O6) cluster results are compared to explore metal-metal bonding and the convergence of the cluster approach. Both atomic and Nb-O and O-O bonding densities are represented by spherical-harmonic expansions and their influence on the Compton profile is discussed. The cluster energy-level spectrum is compared with recent linearized-augmented-plane-wave results of Posternak et al. and with optical data. X-ray emission and absorption data are compared with results of relativistic free-ion transition-state calculations and with cluster results.