A Quantum Chemistry Approach to Linear Vibro-Polaritonic Infrared Spectra with Perturbative Electron–Photon Correlation

In the vibrational strong coupling (VSC) regime, molecular vibrations and resonant low-frequency cavity modes form light–matter hybrid states, vibrational polaritons, with characteristic infrared (IR) spectroscopic signatures. Here, we introduce a molecular quantum chemistry-based computational scheme for linear IR spectra of vibrational polaritons in polyatomic molecules, which perturbatively accounts for nonresonant electron–photon interactions under VSC. Specifically, we formulate a cavity Born–Oppenheimer perturbation theory (CBO-PT) linear response approach, which provides an approximate but systematic description of such electron–photon correlation effects in VSC scenarios while relying on molecular ab initio quantum chemistry methods. We identify relevant electron–photon correlation effects at the second order of CBO-PT, which manifest as static polarizability-dependent Hessian corrections and an emerging polarizability-dependent cavity intensity component providing access to transmission spectra commonly measured in vibro-polaritonic chemistry. Illustratively, we address electron–photon correlation effects perturbatively in IR spectra of CO 2 and Fe(CO) 5 vibro-polaritonic models in sound agreement with nonperturbative CBO linear response theory.