Preparing initial conditions for excited-state dynamics in solvated systems

Performing excited-state dynamics or computing spectra on solvated systems requires sampling the ground state to generate initial conditions. Initial conditions (or snapshots for spectra) are typically produced by QM/MM Boltzmann sampling (following MM equilibration or optimization). It is now becoming clear that one should wait for a set period of time, which we call the healing time, before beginning to sample from the QM/MM trajectory. An appropriate healing time should be as short as possible (to avoid unnecessary computational effort), but long enough to equilibrate to the QM/MM ground state distribution. Healing times in previous studies range from tens of femtoseconds to tens of picoseconds, suggesting the need for guidelines to choose a healing time. We examine the effect of healing trajectory length on the nonadiabatic dynamics and spectrum of a first-generation Donor-Acceptor Stenhouse Adduct in chloroform. Insufficient healing times skew the branching ratio of relaxation back to the ground state and alter the relaxation time for one pathway. The healing time exerts a weaker influence on the spectrum: an insufficient healing time may cause a slight red shift. Monitoring the solute temperature during the healing trajectory provides a reasonable estimate for the healing time. Last, we apply the solute temperature metric to other common equilibration methods for solvated systems.