  Relativistic correction to the Coulomb interaction is considered for strongly
correlated electron orbitals. The atomic representation of the Coulomb-Breit
interaction and its physical origin are clarified, to generalize a concept of
the Slater-Condon parameters. In the derivation, it proves advantageous to
employ the spherical representation of photons, which can be categorized into
three types: scalar, electric, and magnetic photons. The electronic degrees of
freedom can similarly be classified in terms of the angular momentum tensor or
multipoles. Consequently, the interaction between electric multipoles is
mediated by scalar photons, magnetic multipoles by magnetic photons, and
magnetic toroidal multipoles by electric photons. These parameters can be
integrated with the multiorbital Hubbard model or the Anderson lattice and
serve as the foundational framework for investigating relativistic strongly
correlated electron systems.
