  This perspective provides a brief introduction into the theoretical
complexity of polaritonic chemistry, which emerges from the hybrid nature of
strongly coupled light-matter states. To tackle this complexity, the importance
of ab initio methods is highlighted. Based on those, novel ideas and research
avenues are developed with respect to quantum collectivity, as well as for
resonance phenomena immanent in reaction rates under vibrational strong
coupling. Indeed, fundamental theoretical questions arise about the mesoscopic
scale of quantum-collectively coupled molecules, when considering the
depolarization shift in the interpretation of experimental data. Furthermore,
to rationalise recent findings based on quantum electrodynamical
density-functional theory (QEDFT), a simple, but computationally efficient,
Langevin framework is proposed, based on well-established methods from
molecular dynamics. It suggests the emergence of cavity induced non-equilibrium
nuclear dynamics, where thermal (stochastic) resonance phenomena could emerge
in the absence of external periodic driving. Overall, we believe the latest ab
initio results indeed suggest a paradigmatic shift for ground-state chemical
reactions under vibrational strong coupling, from the collective quantum
interpretation towards a more local, (semi)-classically and non-equilibrium
dominated perspective. Finally, various extensions towards a refined
description of cavity-modified chemistry are introduced in the context of QEDFT
and future directions of the field are sketched.
