Ab Initio Linear-Response Approach to Vibro-Polaritons in the Cavity Born–Oppenheimer Approximation

Abstract

Recent years have seen significant developments in the study of strong light–matter coupling including the control of chemical reactions by altering the vibrational normal modes of molecules. In the vibrational strong coupling regime, the normal modes of the system become hybrid modes which mix nuclear, electronic, and photonic degrees of freedom. First-principles methods capable of treating light and matter degrees of freedom on the same level of theory are an important tool in understanding such systems. In this work, we develop and apply a generalized force constant matrix approach to the study of mixed vibration-photon (vibro-polariton) states of molecules based on the cavity Born–Oppenheimer approximation and quantum-electrodynamical density-functional theory. With this method, vibro-polariton modes and infrared spectra can be computed via linear-response techniques analogous to those widely used for conventional vibrations and phonons. We also develop an accurate model that highlights the consistent treatment of cavity-coupled electrons in the vibrational strong coupling regime. These electronic effects appear as new terms previously disregarded by simpler models. This effective model also allows for an accurate extrapolation of single and two molecule calculations to the collective strong coupling limit of hundreds of molecules. We benchmark these approaches for single and many CO 2 molecules coupled to a single photon mode and the iron pentacarbonyl Fe(CO) 5 molecule coupled to a few photon modes. Our results are the first ab initio results for collective vibrational strong coupling effects. This framework for efficient computations of vibro-polaritons paves the way to a systematic description and improved understanding of the behavior of chemical systems in vibrational strong coupling.

Polaritonic Chemistry from First Principles via Embedding Radiation Reaction

“…This allows us to relax previous simplifications to only single or few molecules with artificially increased coupling strength. 22 , 51 , 53 , 68 Already our here illustrated simple proton-tunneling model exhibited nontrivial behavior with the number of emitters. We can expect that complex multistep chemical reactions will further complicate this trend—a strong argument for the need of first-principles techniques.…”

“…Descriptions from first-principles have been restricted to small numbers of molecules. 47 − 53 Notable representatives are quantum-electrodynamical density-functional theory (QEDFT) 54 − 57 and cavity coupled-cluster theory. 58 , 59 …”

“…The vast majority of theoretical studies are therefore based on simplified models that possess a limited applicability to complex systems and chemical reactions. Descriptions from first-principles have been restricted to small numbers of molecules. − Notable representatives are quantum-electrodynamical density-functional theory (QEDFT) − and cavity coupled-cluster theory. , …”

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

“…50,92 This effect is also anticipated for vibrational strong coupling, which can be described by ab initio linear response theory in a similar fashion. 51 However, for the moment, the computational verification remains an open research question. The environment investigated in Ref.…”

Collective response in light-matter interactions: The interplay between strong coupling and local dynamics

“…[11,91] and the recent cavity-BO based study of Ref. [92]) provide important insights, but a fundamental understanding of the collective dynamics involving cavity polaritons is still lacking, making complete interpretation of experimental observations difficult.…”