Julien Eng | Searcharxiv

arXiv2026

Perspective on a challenge: predicting the photochemistry of cyclobutanone

This Perspective is part of a Special Topic that explored the maturity of nonadiabatic molecular dynamics for predicting photochemical processes. In 2023, a prediction challenge was issued to the community of computational photochemists to simulate the photochemistry of cyclobutanone, photoexcited at 200 nm, and the resulting time-resolved MeV-UED signal. The challenge attracted 15 theoretical predictions from more than 70 researchers, employing a wide range of strategies for electronic structure and nonadiabatic molecular dynamics to predict the time-resolved MeV-UED signal before the experiment had been conducted at SLAC (Stanford, USA). The MeV-UED instrument at Shanghai Jiao Tong University was also used to provide a second independent time-resolved MeV-UED signal for the photochemistry of cyclobutanone. This Perspective discusses the various approaches and strategies used by the participants to predict the photochemistry of cyclobutanone. This work also summarizes the strengths and weaknesses of various methods used for photoexcitation, electronic structure, nonadiabatic dynamics, and calculation of observables, as agreed by the participants during a CECAM workshop dedicated to the results of the challenge and organized in Lausanne in April 2025. This Perspective also collects all the predicted time-resolved MeV-UED signals into a single figure, together with the experimental signal. This challenge (i) demonstrated the qualitative predictive power of nonadiabatic molecular dynamics and (ii) underscore the impact of electronic-structure theory on the outcome of the excited-state dynamics and the need for its careful benchmarking. This effort allowed the community to share practical strategies to perform nonadiabatic dynamics (discussed in the present Perspective) and constitutes a 'calibration' exercise for computational photochemistry.

physics.chem-ph↗

arXiv2024

The Photochemistry of Rydberg Excited Cyclobutanone: Photoinduced Processes and Ground State Dynamics

Owing to ring-strain, cyclic ketones exhibit complex excited-state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited-state dynamics of cyclobutanone after $\pi^{\ast}\leftarrow n$ excitation into the lowest-energy excited singlet state (S$_1$) has been extensively studied, the dynamics following 3$s\leftarrow n$ excitation into the higher-lying singlet Rydberg (S$_2$) state are less well understood. Herein, we couple quantum and excited-state trajectory surface-hopping molecular dynamics simulations to study the relaxation of cyclobutanone following 3s$\leftarrow n$ excitation and to predict the ultrafast electron diffraction scattering signal that we anticipate to arise from the relaxation dynamics that we observe. Our simulations indicate that relaxation from the initially-populated singlet Rydberg state occurs on the hundreds-of-femtosecond to picosecond timescale consistent with the symmetry-forbidden nature of the state-to-state transition involved. Once cyclobutanone has relaxed non-radiatively to the electronic ground state (S$_0$), the vibrationally hot molecules have sufficient energy to form multiple fragmentory products on the electronic ground-state surface including C$_2$H$_4$ + CH$_2$CO (C2; 20%), and C$_3$H$_6$ + CO (C3; 2.5%). We discuss the limitations of our simulations, how these may influence the outcome of the excited-state dynamics we observe, and -- ultimately -- the predictive power of the simulated experimental observable.

physics.chem-ph↗