Relativistic theory for indirect nuclear spin–spin couplings within the polarization propagator approach

We present a relativistic theory for the nuclear spin–spin coupling tensor within the polarization propagator approach using the particle‐hole Dirac–Coulomb–Breit Hamiltonian and the full four‐component wave function. We give explicit expressions for the coupling tensor in the random‐phase approximation, neglecting the Breit interaction. A purely relativistic perturbative electron–nuclear Hamiltonian is used and it is shown how the single relativistic contribution to the coupling tensor reduces to Ramsey's three second‐order terms (Fermi contact, spin–dipole, and paramagnetic spin–orbit) in the nonrelativistic limit. The principal propagator becomes complex and the leading property integrals mix atomic orbitals of different parity. The well‐known propagator expressions for the coupling tensor in the nonrelativistic limit is obtained neglecting terms of the order c−n (n ⩾ 1).