  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
non-perturbative methods within the frameworks of quantum mechanics (QM) as
well as quantum electrodynamics (QED). 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 non-retarded
intermolecular interactions. The obtained intermolecular energy contributions
are classified as field-induced (FI) electrostatics, FI 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 combining four complementary 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.
