Deciphering the Influence of Ground-State Distributions on the Calculation of Photolysis Observables

Nonadiabatic molecular dynamics offers a powerful toolfor studyingthe photochemistry of molecular systems. Key to any nonadiabatic moleculardynamics simulation is the definition of its initial conditions (ICs), ideally representing the initial molecular quantum stateof the system of interest. In this work, we provide a detailed analysisof how ICs may influence the calculation of experimental observablesby focusing on the photochemistry of methylhydroperoxide (MHP), thesimplest and most abundant organic peroxide in our atmosphere. Weinvestigate the outcome of trajectory surface hopping simulationsfor distinct sets of ICs sampled from different approximate quantumdistributions, namely harmonic Wigner functions and ab initio moleculardynamics using a quantum thermostat (QT). Calculating photoabsorptioncross-sections, quantum yields, and translational kinetic energy mapsfrom the results of these simulations reveals the significant effectof the ICs, in particular when low-frequency (& SIM; a few hundredcm(-1)) normal modes are connected to the photophysicsof the molecule. Overall, our results indicate that sampling ICs fromab initio molecular dynamics using a QT is preferable for flexiblemolecules with photoactive low-frequency modes. From a photochemicalperspective, our nonadiabatic dynamics simulations offer an explanationfor a low-energy tail observed at high excitation energy in the translationalkinetic energy map of MHP.