  Nonadiabatic molecular dynamics offers a powerful tool for studying the
photochemistry of molecular systems. Key to any nonadiabatic molecular dynamics
simulation is the definition of its initial conditions, ideally representing
the initial molecular quantum state of the system of interest. In this work, we
provide a detailed analysis of how initial conditions may influence the
calculation of experimental observables by focusing on the photochemistry of
methylhydroperoxide, the simplest and most abundant organic peroxide in our
atmosphere. We investigate the outcome of trajectory surface hopping
simulations for distinct sets of initial conditions sampled from different
approximate quantum distributions, namely harmonic Wigner functions and ab
initio molecular dynamics using a quantum thermostat. Calculating
photoabsorption cross-sections, quantum yields, and translational kinetic
energy maps from the results of these simulations reveals the significant
effect of the initial conditions, in particular when low-frequency (~ a few
hundred cm-1) normal modes are connected to the photophysics of the molecule.
Overall, our results indicate that sampling initial conditions from ab initio
molecular dynamics using a quantum thermostat is preferable for flexible
molecules with photoactive low-frequency modes. From a photochemical
perspective, our nonadiabatic dynamics simulations offer an explanation for a
low-energy tail observed at high excitation energy in the translational kinetic
energy map of methylhydroperoxide.
