  Computer simulation has long been an essential partner of ultrafast
experiments, allowing the assignment of microscopic mechanistic detail to
low-dimensional spectroscopic data. However, the ability of theory to make a
priori predictions of ultrafast experimental results is relatively untested.
Herein, as a part of a community challenge, we attempt to predict the signal of
an upcoming ultrafast photochemical experiment using state-of-the-art theory in
the context of preexisting experimental data. Specifically, we employ ab initio
Ehrenfest with collapse to a block (TAB) mixed quantum-classic simulations to
describe the real-time evolution of the electrons and nuclei of cyclobutanone
following excitation to the 3s Rydberg state. The gas-phase ultrafast electron
diffraction (GUED) signal is simulated for direct comparison to an upcoming
experiment at the Stanford Linear Accelerator Laboratory. Following initial
ring-opening, dissociation via two distinct channels is observed: the C3
dissociation channel, producing cyclopropane and CO, and the C2 channel,
producing CH$_2$CO and C$_2$H$_4$. Direct calculations of the GUED signal
indicate how the ring-opened intermediate, the C2 products, and the C3 products
can be discriminated in the GUED signal. We also report an a priori analysis of
anticipated errors in our predictions: without knowledge of the experimental
result, which features of the spectrum do we feel confident we have predicted
correctly, and which might we have wrong?
