Theoretical study of the spectroscopic constants of the ground state of the diatomic Ba-RG (RG = Kr, Xe, Rn) based on the coupled cluster theory with spin–orbit coupling

The spectroscopic constants including equilibrium distance, harmonic frequency and binding energy of the ground state of the diatomic Ba-RG (RG = Kr, Xe, Rn) are studied by using the closed-shell coupled-cluster theory with spin–orbit coupling (SOC) at the singles, doubles, and non-iterative triples level [CCSD(T)] based on the two-component relativistic pseudo-potentials. The advantage of the adopted computational protocol is that the SOC is incorporated in the post-Hartree–Fock part (i.e. the couple-cluster iteration) which makes it possible to significantly improve the computational efficiency. The extrapolation to the complete basis set (CBS) limit is used to provide the most accurate computational values in the framework of the adopted theoretical approach. The computational values to the CBS limit show that the SOC effect decreases the equilibrium distance by 0.067 Å while the binding energy increases by 21.023 cm −1 for the heaviest Ba-Rn, but not significant in the Ba-Kr and Ba-Xe. To date, both experimental and theoretical spectroscopic constants for Ba-Rn are unavailable, the present work thus provides the reliable theoretical results of the ground state of Ba-Rn for the future investigations.