  We formulate a non-relativistic quantum field theory to model interactions
between quantized electromagnetic fields and localized charge-current
distributions. The electronic degrees of freedom are encoded in microscopic
polarization and magnetization field operators whose moments are identified
with the multipole moments of the charge-current distribution. The multipolar
Hamiltonian is obtained from the minimal coupling Hamiltonian through a unitary
transformation, often referred to as the Power-Zienau-Woolley transformation;
we renormalize this Hamiltonian using perturbation theory, the result of which
is used to compute the leading-order radiative corrections to the electronic
energy levels due to interactions between the electrons and quantum vacuum
fluctuations in the electromagnetic field. Our renormalized energy shift
constitutes a generalization of the Lamb shift in atomic hydrogen, valid for
general localized assemblies of atoms and molecules, possibly with net charge
but absent free current. By expanding the fields in a series of multipole
moments, our results can be used to study contributions to this energy shift
coming from specific multipole moments of arbitrary order.
