Nonadiabatic Excited State Molecular Dynamics: Perspectives for a Robust Future

The simulation of nonadiabatic molecular dynamics is an indispensable tool for understanding complex ultrafast photoinduced processes such as charge and energy transfer, and nonradiative relaxation. We have developed a computationally efficient nonadiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions among multiple adiabatic excited state potential energy surfaces (PESs) using the fewest-switches surface hopping (FSSH) algorithm. The NA-ESMD methodology allows for simulation of nonadiabatic dynamics in large molecular systems with hundreds of atoms on ~10 ps time scales where multiple coupled excited states are involved. From these calculations, we can learn about energy relaxation (vibrational and electronic) and transfer rates, details of the nonadiabatic couplings and their relationship with molecular motion, and spectroscopic signatures. The results of NA-ESMD simulations can vary drastically depending on the chosen parameters for propagation and adoption of various approximation schemes. Furthermore, neglecting to treat trivial unavoided crossings between excited state PESs can cause quantum transitions to be missed due to the finite valued propagation time step and the strongly localized nonadiabatic couplings. This failure can result in unphysical long-range energy transfer, even between two non-interacting molecules separated by large distances. Here we analyze the considerations involved in selecting parameters, the validity of commonly adopted approximations, and their effects on the simulated dynamics. In order to avoid spurious artifacts arising from state crossings, we have developed an algorithm to detect trivial unavoided crossings between excited state PESs. As we will demonstrate in this work, implementation of this novel method creates an improved NA-ESMD framework that can now be used to simulate systems involving numerous crossings between multiple excited state PESs. The final ingredient for a robust NA-ESMD approach is the treatment of long-lived quantum coherences. In this work, we will evaluate the performance of two computationally low-cost methods for incorporating quantum decoherence into nonadiabatic dynamics. Ultimately, the application of methods developed and tested for small systems involving only a few excited states to large polyatomic systems is not straightforward. The automatic extrapolation to multi-state simulations can be misleading and their validity must be investigated.