Noteworthy, this does not mean that the self-assembly is less favored in the BTA system than in the M model -the probability associated to larger assemblies is rather similar in the two systems (e.g., ∼ 73 % for 17 -32 monomers in the M system with ε = 40 kJ mol -1 vs. ∼ 69 % in the BTA system at T = 340 K). In conditions where monomer self-assembly is more favored (e.g. M with ε = 50 kJ mol -1 and BTA at T = 300 K), large fibre sizes are even more persistent in time in the BTA than in the M system, while the same holds also for the monomers. This suggests that the free-energy landscape for monomers self-assembly differs between the BTA and the M system. In particular, consistent with the experimental evidence, the BTA systems show the characteristic behavior of cooperative self-assembly, where a critical-size nucleus must be overcome to self-assembly, and where the presence of monomers along with longer fibers is expected at the equilibrium. In this perspective, the assembly-size distribution in Fig. and probability matrices in Figure ,b depict M as a predominantly isodesmic system. The increased survival of disassembled monomers can be imputed to the flexible structure of the BTA monomer model. In such a higher-resolution model, the side-chains can wrap around the BTA centers, minimising the residual solvophobicity of the monomer (cores), and generating a globular and isotropic solvated state of the monomers. Such effect is absent in the M monomers due to their flat and rigid structure. Such a difference between the two models becomes less relevant when aggregates are considered (negligible for size > 3 -5 monomers), as in larger BTA assemblies the monomers can optimise their interactions collectively, forming ordered conformations in which the side-chains are extended and do not interfere with the core-core interactions. Such evidence allows us to draw the following considerations. First, in such systems the cooperativity in the self-assembly seems to emerge from competing effects: the interaction between the monomers, favoring self-assembly, vs. the flexibility of the molecular structure, which determines differences between the configuration of monomers in the disassembled and in the assembled states, due to molecular movements and to the presence of residual (even minimal) solvophobic effects. Second, in the case of rather complex and flexible monomers, a minimalistic description of their structure may determine an over-approximated representation of self-assembly, impairing a correct modeling of cooperativity in self-assembly.