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 approximate excitation schemes (e.g., vertical excitation by an implicit 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, coherently excited, electronic populations are then used to stochastically select the initial electronic states in a postprocessing 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. The new scheme is made freely available as part of the SHARC 4 dynamics package. We illustrate and validate the new excitation scheme by means of several simulations of sodium iodide and 6-cyanobenzquinuclidine excited with laser pulses of different energy and pulse duration, comparing to quantum dynamics results.