Quantum framework for describing retarded and nonretarded molecular interactions in external electric fields

We employ various quantum-mechanical approaches for studying the impact of electric fields on both nonretarded and retarded noncovalent interactions between atoms or molecules. To this end, we apply perturbative and nonperturbative methods within the frameworks of quantum mechanics as well as quantum electrodynamics. In addition, to provide a transparent physical picture of the different types of resulting interactions, we employ a stochastic electrodynamic approach based on the zero-point fluctuating field. Atomic response properties are described via harmonic Drude oscillators-an efficient model system that permits an analytical solution and has been convincingly shown to yield accurate results when modeling nonretarded intermolecular interactions. The obtained intermolecular energy contributions are classified as field-induced electrostatics, field-induced polarization, and dispersion interactions. The interplay between these three types of interactions enables the manipulation of molecular dimer conformations by applying transversal or longitudinal electric fields along the intermolecular axis. Our framework based on the combination of four different theoretical approaches paves the way toward a systematic description and improved understanding of molecular interactions when molecules are subject to both external and vacuum fields.