Semi-relativistic charge-current density operator
  The charge-current density and two-photon operators consistent with a single-particle semi-relativistic Hamiltonian are derived within a suitable functional derivative formalism which preserves gauge invariance. An application to electron scattering is presented and results are compared with a fully relativistic case and the non-relativistic cases corrected through fourth order in M^{-1}. 
S. Boffi, F. Capuzzi, P. Demetriou and M. Radici