AbstractThe low‐lying electronic states of NiH and NiAt are investigated by using multireference second‐order perturbation theory with relativistic effects taken into account. The potential energy curves as well as the corresponding spectroscopic constants are reported. The results are grossly in good agreement with the available experimental data and should thus be very useful for guiding future experimental measurements. A cross comparison with other nickel monohalides NiX (X = F, Cl, Br, and I) reveals that the change in the spin–orbit splittings when going from lighter to heavier ligands results more from the state interaction than from the relativistic effects of the ligands. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007