The NRQED Hamiltonian and photon-exchange interaction up to $m{\alpha }^8$ order

We derive the effective Hamiltonian of the Nonrelativistic Quantum Electrodynamic up to $m{\alpha }^8$ by using scattering matching approach. At $m{\alpha }^6$ order, these results are coincide with Pachucki’s, which is obtained by applying Foldy-Wouthuysen transformation. And by using the NRQED Hamiltonian, we derive the photon-exchange interaction in non-retarded approximation and the retardation correction up to $m{\alpha }^8$. The energy shift of the photon-exchange interaction is obtained by studying the pole of the total Green function.