  We have revisited the valence band electronic structure of NiO by means of
hard x-ray photoemission spectroscopy (HAXPES) together with theoretical
calculations using both the GW method and the local density approximation +
dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity
problem in DMFT is solved through the exact diagonalization (ED) method. We
show that the LDA+DMFT method alone cannot explain all the observed structures
in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and
p derived states to properly position their binding energies. Our results
establish that a combination of the GW and DMFT methods is necessary for
correctly describing the electronic structure of NiO in a proper ab-initio
framework. We also demonstrate that the inclusion of photoionization cross
section is crucial to interpret the HAXPES spectra of NiO.We argue that our
conclusions are general and that the here suggested approach is appropriate for
any complex transition metal oxide.
