The electronic structure of NiO is investigated by means of a series of calculations for a ${\mathrm{NiO}}_{6}$${\mathrm{}}^{10\mathrm{\ensuremath{-}}}$ cluster with use of the local-spin-density-functional theory. Energies of transitions relevant to photoemission and optical excitation are obtained with use of the transition-state method. Ionization potentials and electron affinities are obtained and used to evaluate the Hubbard electron interaction parameter U, which is found to be in the range of 1.5--3.3 eV depending on the manner of its definition.