Benchmarking Electronic-Structure Methods for the Description of Dark Transitions in Carbonyls at and Beyond the Franck–Condon Point

Herein, we propose
a comprehensive benchmark of electronic-structure
methods to describe dark transitions, that is, transitions to excited
electronic states characterized by a near-zero oscillator strength.
This type of electronic state is particularly important for the photochemistry
of molecules containing carbonyl groups, such as atmospheric volatile
organic compounds (VOCs). The oscillator strength characterizing a
dark transition can change dramatically by a slight alteration of
the molecular geometry around its ground-state equilibrium, the so-called
non-Condon effects. Hence, testing the performance of electronic-structure
methods for dark transitions requires considering molecules at their
Franck–Condon point (i.e., equilibrium geometry), but also
beyond the Franck–Condon point. Our benchmark focuses on various
electronic-structure methodsLR-TDDFT(/TDA), ADC(2), CC2, EOM-CCSD,
CC2/3, XMS-CASPT2with CC3/aug-cc-pVTZ serving as a theoretical
best estimate. These techniques are tested against a set of 16 carbonyl-containing
VOCs at their equilibrium geometry. We then assess the performance
of these methods to describe the dark transition of acetaldehyde beyond
its Franck–Condon point by (i) distorting the molecule toward
its S1 minimum energy structure and (ii) sampling an approximate
ground-state quantum distribution for the molecule and calculating
photoabsorption cross-sections within the nuclear ensemble approach.
Based on the calculated cross-sections, we calculate the photolysis
half-life as depicted by the different electronic-structure methodshighlighting
the impact of the different electronic-structure methods on predicted
experimental photolysis observables. The observed inhomogeneities
in the performance of certain methods in different regions of the
potential energy surface, and their effect on the calculated observables,
highlight the need to conduct analyses beyond the Franck–Condon
point when benchmarking electronic-structure methods for describing
excited states.