To benchmark our calculations, we have compared the MS-CASPT2 calculated energies of the S0 and S1 potential energy surfaces at some of the key conformations with those for a related, neutral, fluorene-based molecular motor, 9-(2,4,7-trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene.26 On the S0 potential energy surface, conformer 1b of our molecular motor is 0.31 eV higher in energy than conformer 1a. This energy difference is slightly larger than the equivalent energy difference calculated for the neutral, fluorene-based molecular motor, using DFT with the B3LYP exchange correlation functional and 6-31G* basis set (0.15 eV).26