  Molecular polaritons arise when molecules interact so strongly with light
that they become entangled with each other. This light-matter hybridization
alters the chemical and physical properties of the molecular system and allows
chemical reactions to be controlled without the use of external fields. We
investigate the impact of strong light-matter coupling on the electronic
structure using perturbative approaches and demonstrate that
Rayleigh-Schr\"odinger perturbation theory can reproduce the ground state
energies in optical cavities to comparable accuracy as ab initio cavity quantum
electrodynamics methodologies for currently relevant coupling strengths. The
method is effective in both low and high cavity frequency regimes and
straightforward to implement via response functions. Furthermore, we establish
simple relations between cavity-induced intermolecular forces and van der Waals
forces. These findings provide valuable insight into the manipulation of
ground-state polaritonic energy landscapes, shedding light on the systems and
conditions in which modifications can be achieved.
