Direct wavepacket dynamics with spin–orbit coupling: simulation of thioformaldehyde

The non-radiative photophysical phenomenon known as intersystem crossing is simulated, for the first time, in fully coupled direct nuclear wavepacket dynamics based on Gaussian basis functions (DD-vMCG method). As a case-study, the ultrafast photoinduced dynamics of thioformaldehyde is studied using this method. Working in the spin-diabatic basis, the simulations presented here incorporate the effect of spin-orbit coupling, thus allowing the transfer of population density between states of different spin multiplicities (in this case, singlets and triplets). Even though no significant intersystem crossing is expected in the first ps after exciting the first singlet excited state of thioformaldehyde, our calculations show that the subtle effects of non-negligible spin-orbit coupling elements are accounted for. Additionally, internal conversion back to the ground state is observed, mediated by non-adiabatic effects. While our reference simulations are based on MS-CASPT2 and MRCI calculations, we also investigate the sensitivity of the dynamics to different choices of underlying electronic structure methods. Finally, a comparison is made between results presented here, and those obtained from previously published surface-hopping simulations, commenting on possible methodological reasons for any discrepancies.