  Diagrammatic techniques are well-known in the calculation of dispersion
interactions between atoms or molecules. The multipolar coupling scheme
combined with Feynman ordered diagrams significantly reduces the number of
graphs compared to elementary stationary perturbation theory. We review
calculations of van der Waals-Casimir-Polder forces, focusing on two atoms or
molecules one of which is excited. In this case, calculations of the
corresponding force are notorious for mathematical issues connected to the
spontaneous decay of the excitation. Treating such unstable states in a full
non-equilibrium theory provides a physical interpretation of apparent
contradictions in previous results and underlines the importance of decay
processes for the intermolecular potential. This may have important
implications on reactions in biological systems, where excited states may be
relatively long-lived and the resonant intermolecular force may result in
directed Brownian motion.
