Solvent Effects on Nonadiabatic Dynamics: Ab Initio Multiple Spawning Propagated on CASPT2/xTB Potentials.

An approach to simulate nonadiabatic dynamics in solution is introduced, which relies on the propagation of the nuclear wavepacket with the Ab Initio Multiple Spawning (AIMS) method under the effect of potential energy calculated with a hybrid but fully quantum mechanical scheme (QM/QM'). The electronic energies of the excited states of the chromophore are calculated with multireference perturbation theory (CASPT2), and the embedding molecules are described with a tight binding Hamiltonian (GFN2-xTB). This implementation is fully open source and relies on the combination of PySpawn, OpenMolcas, and xTB. Additionally, ORCA is used to properly generate the initial conditions in solution, showing how the combination of cutting-edge implementations in several commonly used software can push the state of the art of nonadiabatic dynamics in solution toward a new high standard of accuracy. The dynamics of ethylene in vacuum, in acetone, and in chloroform is reported as a test case, with a detailed analysis of the AIMS runs that shows important geometrical and electronic effects of the solvents on the decay mechanism of the chromophore.