A CECAM workshop took place from April 8 to 10 in Lausanne (Switzerland), co-organized by two members of the UPDICE consortium, Basile Curchod and Graham Worth, together with Federica Agostini (Paris-Saclay), Todd Martínez (Stanford), and Sara Bonella (CECAM-EPFL).

This workshop aimed to discuss the results of a recent prediction challenge addressed to the nonadiabatic dynamics community. The challenge consisted of predicting the photochemistry of cyclobutanone and the corresponding MeV-UED experimental signal and submitting an article on these results *before* the experiment was conducted at SLAC (Stanford). Fifteen articles were published in the Journal of Chemical Physics in response to this challenge (https://pubs.aip.org/collection/16531/Prediction-Challenge-Cyclobutanone-Photochemistry). The goal of this CECAM workshop was to discuss the results of this community challenge and agree on what worked and what did not work, building a roadmap for future developments in the field of nonadiabatic dynamics. Alice Green (Edinburgh) and Andrew Orr-Ewing (Bristol, lead of UPDICE) represented the experimental side of this challenge and supported the discussion on the calculation of experimental observables.