We will discuss instanton theory, the simplest approach for rigorously describing quantum tunnelling and present results of calculations on various molecular systems. This semiclassical theory uniquely defines and locates an optimal tunnelling pathway in full dimensionality, which is identified as the reaction mechanism. As well as this mechanistic insight, the theory offers an efficient approach for predicting tunnelling splittings and low-temperature rate constants. We have recently made this method much more efficient using machine-learning tools to interpolate an ab initio potential-energy surface along the tunnelling pathway. An extension to include the geometric-phase effect allows us to explain the high-resolution experimental spectrum of the methane cation and its isotopologues. Contrary to expectation, the overall amount of deuteration does not necessarily reduce the tunnelling splitting. We will also present applications to concerted proton transfer and other hydrogen-bond rearrangements in water clusters of increasing size, all the way up to an ice crystal.

Quantum tunnelling and the geometric-phase effect in molecules and the condensed phase

2019 Fall National Meeting

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