Hyperfine interactions between electrons

The relativistic Breit Hamiltonian between electrons is transformed into an effective vector potential ${\mathbf{A}}_i$ for the $i\mathrm{.}$th electron, ${\mathbf{A}}_i$ having the structure of a recoil–corrected hyperfine operator. Apart from a small three–body operator, the Dirac–Breit equation is now easier applied to relativistic magnetic properties of complex systems.