The NEB-TS algorithm (a climbing-image nudged elastic band calculation, followed by transition state optimization) was used to locate transition states (TS) between different conformers of the pyrene excimer in the S1 state at the TD-DFT level. The first-order saddle-point nature of the optimized TS structures was checked by the presence of a single imaginary frequency in the vibrational spectrum and by the consistency of the corresponding normal mode with the associated reaction pathway. When the TS optimization did not fully achieve tight convergence in a maximum of 500 steps, an earlier structure with low gradients, or even the converged climbing image, was selected as a guess geometry and optimized along the appropriate eigenvector after frequency calculation.