Approximate theoretical model for the five electronic states (Ω = 5/2, 3/2, 3/2, 1/2, 1/2) arising from the ground 3d9 configuration in nickel halide molecules and for rotational levels of the two Ω = 1/2 states in that manifold

► Effective crystal-field and spin-orbit Hamiltonian for NiF and NiCl. ► Effective rotational Hamiltonian for diatomics with d9 configuration. ► Omega-doubling parameter p calculated from crystal-field and spin-orbit splittings.