  Quantum dynamics simulations are becoming a standard tool for simulating
photo-excited molecular systems involving a manifold of coupled states, known
as non-adiabatic dynamics. While these simulations have had many successes in
explaining experiments and giving details of non-adiabatic transitions, the
question remains as to their predictive power. In this work, we present a set
of quantum dynamics simulations on cyclobutanone, using both grid-based
multi-configuration time-dependent Hartree (MCTDH) and direct dynamics
variational multi-configuration Gaussian (DD-vMCG) methods. The former used a
parameterised vibronic coupling model Hamiltonian and the latter generated the
potential energy surfaces on-the-fly. The results give a picture of the
non-adiabatic behaviour of this molecule and were used to calculate the signal
from a gas-phase ultrafast electron diffraction (GUED) experiment.
Corresponding experimental results will be obtained and presented at a later
stage for comparison to test the predictive power of the methods. The results
show that over the first 500 fs after photo-excitation to the S$_2$ state,
cyclobutanone relaxes quickly to the S$_1$ state, but only a small population
relaxes further to the S$_0$ state. No significant transfer of population to
the triplet manifold is found. It is predicted that the GUED experiments over
this time scale will see s signal related mostly to the C-O stretch motion and
elongation of the molecular ring along the C-C-O axis.
