From a theoretical perspective, there have been numerous studies of molecular rotary motors aimed at understanding how both the molecular structure and electronic structure influence the excited state and ground state dynamics. Kazaryan et al. used ensemble DFT (density functional theory) and OM2/MRCI molecular dynamics calculations to explore the potential energy surface of a fluorene based motor.25–27 They found barriers to rotation along the ground electronic state, S0, as well as the excitation energy to the first excited state, S1, using ensemble DFT and PM3 methods and, in agreement with the later experimental observations,22,23 they predicted that a conical intersection between the S0 and S1 states played a key role in the light-induced isomerisation step. Their MD simulations found an average excited state lifetime of 1.4 ps. Torras et al. employed QM/MM (quantum-mechanics/molecular mechanics) calculations to investigate another fluorene based overcrowded chiral alkene molecular motor and its rotation.28 They optimised structures using unrestricted DFT and the perturbative MP2 method and included solvent effects in their QM/MM calculations. They calculated barriers to rotation which were comparable to experiment and also found that the hybridisation of the ethylene axle evolved from being sp2 to sp3 and back again, as the motor rotated, to minimise steric repulsions. CASSCF (complete active space self consistent field) calculations were carried out by Amatatsu using a 2-orbital, 2-electron active space.29,30 Again it was found that a conical intersection between the S1 and S0 states was a key factor in the light-induced isomerisation step. Recently, Pang et al. have reported the results of trajectory surface-hopping dynamics at the semi-empirical OM2/MRCI level investigating the photoisomerisation dynamics;31 they found timescales in reasonable agreement with those measured by Meech and coworkers,22,23 although they suggest that the ‘dark state’ is not a separate electronic state, as suggested by Amirjalayer et al.,24 but a ‘dark region’ of the S1 state.