  We study the electronic structure of Ta2NiSe5 in its low-temperature
semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the
Ta L3 edge. We also investigate the electronic properties of Ta2NiSe5 within
the density-functional theory using the generalized gradient approximation in
the framework of the fully relativistic spin-polarized Dirac linear muffin-tin
orbital band-structure method. While ARPES, dc transport, and optical
measurements indicate that Ta2NiSe5 is a small band-gap semiconductor, DFT
gives a metallic nonmagnetic solution in Ta2NiSe5 . To obtain the
semiconducting ground state in Ta2 NiSe5 we use a self-interaction correction
(SIC) procedure by introducing an orbital-dependent potential Vl into the
Hamiltonian. We investigate theoretically the x-ray absorption spectroscopy
(XAS) and RIXS spectra at the Ni and Ta L3 edges and analyze the spectra in
terms of interband transitions. We investigate the RIXS spectra as a function
of momentum transfer vector Q and incident photon energy. Because Ta2 NiSe5
possesses only fully occupied (Ni 3d and Se 4p) and completely empty (Ta 5d)
shells with the formal valencies Ta5+ (5d0), Ni0 (3d10 ), and Se2- (4p6 ), both
the Ni and Ta L3 RIXS spectra belong to a charge transfer type with
ligand-to-metal excitations.
