Photoswitches, e.g. molecules that undergo reversible changes in their structure and properties upon irradiation with light, offer promising opportunities of steering the conversion of optical-into mechanical energy through powered nanoscale motions. In particular, the combination of photoswitching with controlled directional motion, referred to as light-driven molecular motors, enables fascinating prospects in nanotechnology and molecular engineering. Notable examples of photoswitch-driven nanodevices and molecular machines include the light-control of surface wettability, optoelectronics such as light-powered electrical switches, chemical sensing, and photosensitive medicine. Especially for biological-and material science applications that do not tolerate high-energy input, the absorption of visible light by the photoswitchable chromophores is particularly desirable. Apart from the long-known azobenzenes, a recently emerging class of photoswitches is given by indigoids and particularly hemithioindigo (HTI) (Scheme 1). These compounds feature such desired low-energy absorption bands for electronic excitations together with a good thermal bi-stability, and chemical durability along the switching process. Consequentially, HTIs have found their way into a plethora of applications including molecular machines such as motors, gears, or tweezers, as well as photoresponsive catalysis, supramolecular chemistry, molecular computing, or photopharmacology. Further chemical modifications have been employed to enable HTI attachment to metal surfaces, or to tailor selfassembly. The photoreactions of hemithioindigo-based molecules were extensively studied in the past by both, experimental and theoretical investigations . In the most straight- forward cases, these molecules undergo torsional deformation of the double bond upon visible light excitation, and thus form Z and E isomers after relaxation to the electronic ground state. However, more complex bond rotations of HTIs in the excited state have also been developed recently, notably concomitant single and double bond rotation as initially propsed by Liu and Asato, the Hula Twist photoreaction, or a single bond rotation. Moreover, the kinetics and type of photoreaction depends on the specific environment, such as the choice of the solvent. Both, the groups of de Vivie-Riedle and Maurer found that the photoinduced doublebond isomerization (DBI) in unsubstituted hemithioindigo (scheme 1) is largely characterized by dihedral rotation around the central -C=C-bond. Moreover, depending on stilbene substitution, the molecule may undergo an early-stage pyramidalization for the transition from the bright S1-state to the dark, reactive S2-state with more electron rich arenes requiring less pyramidalization. However, Yang et al. showed by nonadiabatic molecular dynamics simulations that the reactive DBI pathways of hemithioindigo are defined by conical intersections dominated by dihedral torsion with a rather weak degree of pyramidalization. 50 As can be seen by the multitude of HTI applications mentioned above, suitable functionalization of these molecular photoswitches is key for developing more complex and capable functions in the future. For a truly rational design of such advanced capabilities, in-depth mechanistic understanding of the photochemistry of HTIs and the effect of the functionalization is required -both at the electronic and the molecular scale. From the perspective of theory, such characterization may be provided by timedependent density functional theory (TD-DFT) calculations and more sophisticated multireference methods to study the electronic states, whereas molecular dynamics (MD) simulations offer the investigation of atomic motion using classical Newtons dynamics. The key hurdle to MD investigations is however given by the need to appropriately describe the forces and energies of the atomic interactions -both in the electronic ground state and after photoactivation. While the most accurate approach would call for a fully quantum treatment of both electronic and vibrational degrees of motion, the computational costs of such modelling are quite considerable. As a consequence, only models of a few selected degrees of freedom could be subjected to full quantum dynamics simulations at the current state of the art.