An excitation strategy for the initial condition generation for surface hopping trajectories using electron-only dynamics including explicit laser pulses

Nonadiabatic dynamics simulations, e.g., via trajectory surface hopping, are nowadays used regularly to describe various photoinduced phenomena in molecules. For a number of reasons, in the setup of such simulations, the actual photoexcitation process is often described by rather crude and approximative excitation schemes (e.g., vertical excitation by a delta pulse), and more attention is directed to the ensuing dynamics simulations. However, several studies have implied the importance of properly considering the spatial, temporal, and spectral details of the exciting laser pulse. Here, we suggest the “electron-only explicit” (“EOE”) excitation scheme for setting up trajectory surface hopping initial conditions based on an explicit laser pulse at little computational cost. The scheme is based on solving the time-dependent electronic Schrödinger equation including the explicit influence of a laser pulse within the frozen-nuclei approximation. The obtained time-dependent electronic populations are then used to stochastically select the initial electronic states in a post-processing step. Here, the electronic populations are renormalized such that one excites a reasonable fraction of the initial condition even when operating well within the weak-field regime. We illustrate the new excitation scheme, which is available as part of the SHARC dynamics package, by means of several simulation sets of protonated formaldimine (CH2NH2+) excited by different laser pulses.