Low energy excitations in NiO based on a direct Δ-SCF approach

This paper reports calculated energies and electronic structures of nineteen excited states in NiO based on a Δ-SCF approach reported previously for the transitions in NiO and Sr₂CuO₂Cl₂. They are the spin-flip: ; eight : (one electron) ³A_2g  →  ³ T _2g, (two electron) ³A_2g  →  ³ T _1g, ³A_2g  →  a ¹ E _g, ³A_2g  →  ¹ T _2g, ³A_2g  →  ¹ T _1g, ³A_2g  →  b¹ E _g, ³A_2g  →  b¹ T _2g; two O(2p)  →  Ni(3d); seven Ni(3d)  →  O(2p) (including two spin-flip) and the O(2s)  →  Ni(3d) charge transfer excitations, which span an energy range from 0.25 eV to 17.53 eV, and include the fundamental band gap associated with an excitonic O(2p)  →  Ni(3d) transition at 4.23 eV. These are compared to absorption and emission spectra, and previous calculations. In the case of the O(2p)  →  Ni(3d) excitations, comparisons are given for gap and Γ-point energies derived from HF, PBE0, HSE06, B3LYP, B1WC, GGA and LDA one-electron approximations. Finally, the directly calculated Stokes shifts and associated luminescence energies for the two O(2p)  →  Ni(3d) transitions are reported, where the excitonic value is found to be in good agreement with the recently reported experimental value. (paper)