The Earth’s lower atmosphere presents a complex mixture of photochemical processes that influence chemical composition, affecting air quality, human health, and the oxidative capacity of the atmosphere. Such processes are notoriously difficult to study experimentally and key classes of excited-states, such as dark transitions, provide challenges to theoretical attempts. Here, a comprehensive benchmarking effort is proposed to describe dark transitions, that is transitions to excited electronic states characterised by a near-zero oscillator strength. This type of electronic state dominates the photochemistry of molecules containing carbonyl groups and has an oscillator strength that can change dramatically by slight distortions of the molecular geometry. This benchmark focuses on various commonly employed electronic-structure methods – LR-TDDFT(/TDA), ADC(2), CC2, EOM-CCSD, CC2/3 and XMS-CASPT2 – with CC3/aug-cc-pVTZ serving as the theoretical best estimate. The performance of these methods for the determination of excitation energies and oscillator strengths were tested against a set of 16 carbonyl-containing VOCs at their equilibrium geometries. The benchmark framework was then extended to evaluate the performance of the methods for the description of the dark transitions of acetaldehyde beyond and around its Franck-Condon (equilibrium) geometry. The role of including nuclear quantum effects, in conjunction with adequate electronic-structure theory, is then investigated using acetaldehyde as a model system. Here, the performance of CMD, harmonic Wigner, QT, and PI+GLE sampling methodologies are evaluated for their ability to sample the ground state nuclear distribution to produce theoretical photoabsorption cross-sections within the NEA framework. This work further advocates for the exploration of nuclear configuration space away from the Franck-Condon point for the description of dark-state transitions. Overall, this Thesis aims to provide the foundations for the development of a theoretical framework that allows for the accurate description of dark-state transitions and advocates for their inclusion in future studies and atmospheric chemistry models.
Towards the accurate description of dark transitions in carbonyl compounds at and beyond the Franck-Condon point.
Bone, J. (Author). 9 Dec 2025
Student thesis: Master's Thesis › Master of Science by Research (MScR)