Hi, In negf, interactions are considered as self-energies. For electron-photon interaction, a perturbation Hamiltonian is derived and then self-energy is found using coupled-field theory. Now, my question is: is it possible to just include the interaction Hamiltonian as part of the total electronic Hamiltonian (like the way a potential is included inside the Hamiltonian)? I have not seen this being done. Why do we always use self-energy formulation for these interactions? Thanks. -Golam Rabbani