  In this work we reexamine quantum electrodynamics of atomic eletrons in the
Coulomb gauge in the dipole approximation and calculate the shift of atomic
energy levels in the context of Dalibard, Dupont-Roc and Cohen-Tannoudji (DDC)
formalism by considering the variation rates of physical observables. We then
analyze the physical interpretation of the ordering of operators in the dipole
approximation interaction Hamiltonian in terms of field fluctuations and
self-reaction of atomic eletrons, discussing the arbitrariness in the
statistical functions in second order bound-state perturbation theory.
