We now focus on the propagation process in the Y↔B phase transitions. Research has utilized metadynamics simulations to reproduce the B-Y phase transition processes in CsPbI 3 and FAPbI 3 . However, these studies are based on small-scale systems and specific collective variables (CVs). Consequently, the phase transition mechanisms elucidated may not account for the possibility of nucleation and could be biased by the choice of CVs. Unbiased microsecond-scale MD simulations can only reproduce the Y→B phase transition at high temperatures , and not the slower B→Y phase transition at low temperatures (supplementary material, Fig. ).