Photoinduced processes underpin an ever-expanding range of technological applications, such as photovoltaics, sensors, imaging and photocatalysts. Photons offer multiple advantages as low-loss energy carriers but also in the context of matter: providing a window into its quantum nature, triggering and driving transformations as well as externally controlling them. This project is concerned with understanding the inner workings of photoinduced processes in molecular systems using a combination of nonadiabatic dynamics simulations and calculations of their spectroscopic signatures (both electron, optical and X-ray based). We will for instance investigate internal conversion decay pathways in a prototype excited-state hydrogen transfer (ESIHT) system and their competition with intersystem crossing.[1] ESIHT is a one of the fastest chemical reactions and plays a key role in a range of light-induced biological processes and technological applications. In collaboration with leading experimentalists in the field (key collaborators: Thomas A. Wolf and Ruaridh J. G. Forbes, Stanford University) we successfully procured and conducted two LCLS experiments on acetylacetone photoexcited to its pi-pi* state that have resulted in rich sets of time-resolved X-ray absorption and X-ray scattering data. To facilitate interpretation of these experiments, we will perform ab initio multiple spawning simulations in combination with calculations of the spectroscopic observables. Beyond gas-phase systems, we are investigating photoinduced dynamics in condensed phases and photoactive proteins using hybrid QM/MM approaches.[2] The aim of this research is to decipher the implications of the surrounding environment on the excited-state deactivation. We are particularly interested in disentangling the effects of mutations in photo-reversibly switching fluorescent proteins with direct relevance for the development of new biomarkers and optogenetic applications. We have in a series of papers pioneered the use of the full semi-classical light–matter interaction operator in spectroscopic simulations. We have presented the theory and its implementation in a linear response framework both in the nonrelativistic and relativistic regimes.[3] These previous works showed the advantages of using the full interaction operator over conventional truncated approaches, especially at high photon energies in model systems. Further work will establish the use its use in chemically interesting systems as well as its extension to nonlinear spectroscopies. Theory developments and applications will be performed with the relativistic DIRAC package and nonrelativistic VeloxChem program. [1] List et al. Chem. Sci., 2020,11, 4180-4193 [2] List et al. Chem. Sci., 2022, 13, 373; Jones et al. Chem. Sci., 2021, 12, 11347; Jones et al., J. Am. Chem. Soc., 2022, 144,12732 [3] List et al. J. Chem. Phys. 2015, 142, 244111; List et al. Mol. Phys. 2017, 115, 63; List et al. J. Chem. Phys., 2020, 152, 184110; Horn et al. J. Chem. Phys., 2022, 156, 054113