A quantum field theory of polarization propagators

We present a field-theoretical quantum formalism to describe the space-time response of molecular electronic systems to external, particle-number-conserving perturbations. This work continues previous developments in which polarization propagators were obtained from the path-integral formalism, and an effective model to incorporate QED effects was proposed. A central result of the present study is the emergence of hard-core boson quasiparticles as excitations of quantum fields. By establishing a formal correspondence between one-electron excitation and de-excitation operators and virtual hard-core boson-like quasiparticles, we provide a direct link to polarization propagators through the Feynman propagator of the quasiparticle field. We also outline a likely connection between these quasiparticles and the entanglement of localized molecular orbital excitations.