Cigrang, Léon L. E.; (2025) Use and Development of Direct Quantum Dynamical Methods for Photochemistry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Computational chemistry and molecular physics come together in a field known as Quantum Dynamics, which is about studying the time evolution of physical systems at the quantum mechanical level. Even though quantum theory provides an equation that completely describes this evolution, i.e. the Schrödinger equation, there does not exist a tractable way to solve it for the vast majority of molecular systems. This thesis explores that problem by using and developing computational methods which can provide approximate solutions and thereby contribute to a detailed mechanistic understanding of photoinduced reactions. The methods described in this work are based on the propagation of coupled nuclear wavepackets, ideally suited to account for the effects of non-adiabatic phenomena between excited electronic states. They are typically more computationally demanding than many alternatives, but higher accu- racy is achieved in most cases. This is demonstrated in a comprehensive study of the photodissociation reactions of methanol, showing how high-level electronic structure calculations are used in combination with direct dynamics simulations to provide a comprehensive description of the fundamental phenomena that govern the observed reactions of photo-excited molecules. This is followed by the outline of a newly developed procedure that aims to incorporate the effects of explicit atomistic environments into accurate non-adiabatic quantum dynamics calculations. The approach is based on combining the efficiency of classical molecular mechanics calculations, with the accuracy of coupled nuclear wavepacket propagations. Results are shown which demonstrate the potential applicability of this method. Finally, a critical analysis is given of the various steps required to successfully use direct wavepacket methods. The aim here is to remove practical bottlenecks by suggesting possible automations, as well as consider best practices. It is argued that such evaluations will be invaluable in the context of community-wide efforts to progress the field as a whole.

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