The CCSD(T) approach based on two-component relativistic effective core potential with spin-orbit interaction just included in coupled cluster iteration is adopted to study the spectroscopic constants of ground states of Kr-2, Xe-2 and Rn-2 dimers. The spectroscopic constants have significant basis set dependence. Extrapolation to the complete basis set limit provides the most accurate values. The spin-orbit interaction hardly affects the spectroscopic constants of Kr-2 and Xe-2. However, the equilibrium bond length is shortened about 0.013 and the dissociation energy augmented about 18 cm (1) by the spin-orbit interaction for Rn-2 in the complete basis set limit. (C) 2016 Elsevier B.V. All rights reserved.