  We investigate the electronic structure of Nickel sulphide (NiS) in the
millerite phase using electron spectroscopic measurements and band structure as
well as model Hamiltonian calculations. While band structure calculations are
found to be relatively more successful in describing the experimental valence
band spectrum of this highly conducting phase compared to the hexagonal phase
of NiS, cluster calculations including electron correlation effects are found
to be necessary for the description of certain features in the experimental
spectra, indicating importance of correlation effects even in a highly metallic
system. The electronic parameter strengths obtained from these calculations
confirm that the millerite NiS is a highly covalent pd-metal. The comparative
study of hexagonal and millerite forms of NiS, provides the information
concerning the evolution of the spectral function in a pd-metal as a function
of covalency.
