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

Abstract

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 futuredirections of the field are sketched.

Exact Solution for A Real Polaritonic System Under Vibrational Strong Coupling in Thermodynamic Equilibrium: Absence of Zero Temperature and Loss of Light-Matter Entanglement

“…Overall, the theoretically predicted suppression of matter fluctuations d2 z T at temperatures far beyond T 0 confirm from first principles that the equilibrium dynamics of matter can indeed substantially be modified by ro-vibrational strong coupling to the quantized cavity modes, as proposed in Ref. 20 (before reaching the classical limit k B T hω α ). This observation has potential impact on the future development of polaritonic reaction rate theories and non-equilibrium simulation methods, which are crucial for the design of novel cavitymediated reaction processes and for cavity-mediated modifications of the equilibrium ground state in quantum materials.…”

“…For example in terms of a cavity Born-Oppenheimer picture, 56 which treats the nuclei and photons classically, but accounts for the non-classical nature of the field fluctuations. 20 Choosing this semiclassical picture to interpret the physical mechanisms has two interesting consequences. First, it can trivially be generalised to polaritonic ensembles with different matter constituents, e.g.…”

“…In a next step, we investigate how the strong ro-vibrational coupling affects the (vacuum) field mode and matter fluctuations in thermal equilibrium. Reaching a detailed understanding of cavity-modified fluctuations is not only of theoretical interest, but it is of fundamental importance for the emerging fields of polaritonic chemistry and materials science, where modified fluctuations would call for an adaptation of usual molecular dynamics simulations and rate theories 20 . FIG.…”

“…However, it becomes a relevant quantity for the future development of approximations in theoretical models or for simulation methods under cavity-modified thermal equilibrium conditions (e.g. in terms of open quantum systems 20 or within semi-classical cavity Born-Oppenheimer molecular dynamics 20,56 ).…”

“…However, the details of the photon field and an accurate description of the coupled thermal equilibrium are commonly assumed to be irrelevant in this matter-driven perspective and thus T = 0 is commonly assumed. Yet all of which specialized viewpoints seem to be insufficient to explain certain experimental observations, such as the resonance condition for suppressing chemical reactions via strong coupling 10,20 or how strong coupling can influence complex aggregation processes of molecule-metal complexes 21 . Much experimental effort has thus been spent into engineering the matter subsystem to enforce the (unfortunate) theoretical quantum-optical simplifications, e.g., that molecules become effective two-level systems 22,23 .…”