A perspective on ab initio modeling of polaritonic chemistry: The role of non-equilibrium effects and quantum collectivity

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 summarized. Based on those, novel perspectives 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 QEDFT findings, a simple, but computationally efficient, Langevin perspective 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.