Abstract Relativistic effects are significantly important in understanding the electronic structure and spectra of heavy atoms or molecules containing such atoms. Estimation of relativistic effects at the coupled-cluster (CC) level, using the Cowan—Griffin quasi-relativistic many-electron Hamiltonian, is described. The relativistic effects are treated as an external perturbation to the non-relativistic Born—Oppenheimer many-electron Hamiltonian and are evaluated analytically by the “relaxed” density formulation of coupled-cluster (CC) theory. Applicability of the method is demonstrated by calculating relativistic corrections to the ground state energies and dipole moments of diatomic interhalogens. We also present comparisons with relativistic effective potential results.