The present study is devoted to atomic adsorption of Ni on MgO (100) surface and focuses on the magnetic moment of adsorbed atomic Ni in the initial phase of metal deposition. Both cluster and periodic slab models lead to the same description, i.e., they show an almost degeneracy of the magnetic and nonmagnetic states. However, the total energy related properties are strongly dependent on the computational method chosen. At variance with the large difference for the calculated heats of adsorption, the optimized values for the Ni-O distances are remarkably close: 1.8-1.9 Angstrom for the singlet state and larger than 2 Angstrom for the triplet. The paper also presents a detailed analysis of the problems connected to the definition of a proper reference atomic energy in density functional theory approaches.