Unraveling Deep Tunnelling in Spectroscopy with Ab Initio Instanton Theory

Theoretical prediction of reaction rates is crucial in many fields. The widely used classical Transition State Theory (TST) provides a highly effective method for calculating reaction rates, but it cannot describe quantum tunneling. Ring-polymer instanton theory is a rate theory formally similar to TST that incorporates nuclear quantum effects such as tunneling. It can be combined with first-principles methods to understand tunneling phenomena in chemical reactions of larger molecules. Here, I will report on some progress we have made over the past few years in tunneling simulations by integrating instanton theory, high-precision electronic structure calculations, and machine learning-assisted tunneling path optimization algorithms. For example, we theoretically reproduced the significant ¹⁶O/¹⁸O isotope effect observed in matrix isolation spectroscopy experiments, providing strong theoretical support for oxygen atom tunneling. We also simulated the "fingerprints" (tunneling splitting) left by heavy atom and molecular tunneling in molecular rotational spectra. Beyond chemical reactions under the Born-Oppenheimer approximation, instanton theory can also be applied to explore tunneling phenomena in nonadiabatic chemical reactions under the Fermi's Golden Rule limit. I will also discuss our recent developments on understanding tunneling in the Golden Rule regime.