Cavity Quantum Electrodynamic Nuclear–Electronic Orbital Complete Active Space Configuration Interaction

Polaritons offer the possibility of modifying chemical properties through the strong coupling of molecules to photons confined in optical cavities. From a first-principles perspective, understanding how these strong light–matter interactions cause chemical changes requires quantum mechanical descriptions of electrons, nuclei, and photons. Herein, we develop the ab initio cavity quantum electrodynamic nuclear–electronic orbital complete active space configuration interaction (QED-NEO-CASCI) method, in which electrons, specified nuclei, and cavity photons are all treated quantum mechanically. Using this method, we are able to describe hydrogen tunneling under vibrational strong coupling and geometry changes in a hydrogen-bonded complex coupled to a cavity. Under vibronic strong coupling, we observe features in the polaritonic absorption spectrum that correspond to Raman active transitions outside a cavity. We also develop the real-time variant, denoted time-dependent configuration interaction (QED-NEO-TDCI). This method allows us to simulate nonadiabatic hydrogen tunneling dynamics and observe the real-time quantum mechanical transfer of energy between the molecule and cavity photons. These methods provide the ab initio foundation for studying correlated electronic–nuclear–photonic chemical physics.