Calculated O K-edge XAS Spectra of Niobium Oxide Phases using Bethe-Salpeter equation

X-ray absorption spectra (XAS) were calculated for the oxygen K-edge for 20 different phases of Niobium oxides (NbO_x) using the Bethe-Salpeter equation as implemented within the OCEAN code. Fifteen of the NbO_x phases are amorphous, in which nine are stoichiometric Nb₂O₅, and slightly off-stoichiometric containing a variety of different defects. The other five structures are different crystalline phases (with spacegroup): NbO (Pm3m), NbO₂ (P4₂/mnm), N-Nb₂O₅ (C₂/m), M-Nb₂O₅ (I4/mmm), and B-Nb₂O₅ (C₂/c). The data is given in terms of four different folders: (1) VASP POSCARs for amorphous structures (called amorph_POSCARs), (2) VASP POSCARs for crystalline structures (called crystalline_nboxide_structures), (3) OCEAN outputs for amorphous structures (called amorph_U4), and (4) OCEAN outputs for crystalline structures (called cryst_U4). The 'amorph_U4' folder contains subfolders for each structure, and within each subfolder is a 'Results' folder containing the OCEAN input and output files, including individual XAS spectra for each individual oxygen atom in the structure. The organization of the 'cryst_U4' folder structure is the same as the 'amorph_U4' folder.

To calculate the XAS spectra within the OCEAN code, we first performed DFT (using Quantum Espresso) with an energy cut-off of 92 Ryd., and a simplified Hubbard U of 4 eV was applied to the Nb d orbitals. The `O-high' and `Nb-sp' pseudopotentials from PseudoDojo were used. For the BSE, the electron orbitals were down-sampled onto real space grids chosen to match 1 grid point per 1 a.u., the k-point meshes were chosen to exceed 1 grid point per 0.16 a.u.^-1, and the number of conduction bands was set to 0.126 times the unit cell volume (in a.u.³). 
For the screening, electron orbitals were calculated on k-point meshes exceeding 1 grid point per 0.56 a.u.^-1, and the number of conduction bands was set to 0.437 times the unit cell volume (in a.u.³).
A Lorentzian core-hole broadening of 0.07 eV was included, and additional Gaussian broadening of 0.5 eV was applied.