As a matter of fact, clusters 1, 3.2, 5, and 6 would be responsible for an emission decay faster than those experimentally measured for pyrene excimer. This is related to the non-zero value of the twist angle, resulting in a net transition dipole for the aggregate. However, they also feature a significant degree of stacking between monomers, which helps explain why the rearrangement towards the global minimum can proceed without the need to overcome high energy barriers (see also Table ). For all the examined pairs of clusters, the imaginary frequency mode associated with transition state crossing corresponds to a rather loose motion (30-60 cm -1 ), affecting the relative orientation and/or displacement between the two (rigid) monomers without any appreciable intramolecular distortions.