The strength of the attraction between two monomers in the different solvents was estimated from the free energy as a function of intermolecular separation calculated from OPES simulations (Fig. ). Although this free energy was calculated as a function of distance only (i.e. not considering the orientation of the particles, which is important for distinguishing different aggregate geometries), the minimum at ≈4 Å is due almost exclusively to π-stacked structures as it is the only configuration that allows such close packing. The free energy preference for aggregation of a pair of monomers was approximately 3.8 kcal/mol (6.4 k B T ) in the poor solvent, 2.2 kcal/mol (3.7 k B T ) in the intermediate solvent, and 0.9 kcal/mol (1.5 k B T ) in the "good" solvent (k B T at T = 300 K). In the poor solvent, this attraction is sufficiently strong that even chains held together by a single monomer are unlikely to separate often, allowing for the build-up of large aggregates. Additionally, the convergence of ⟨N pair ⟩/N to a value far less than 1 in the poor and intermediate solvents (Fig. ) indicates that with an average N pair of just 8 in the poor solvent, the average number of overlaps neither decreases nor increases with time. This finding again indicates that chains that are much less than fully overlapping are stable for long periods of time in the poorer solvents. A comparison with the same free energy in the AA system can be found in the ESI (Fig. ).