The energy difference between the amorphous and yellow phase CsPbI 3 is estimated at about 55 meV per atom using MD simulations, resulting in a transition barrier of 23 eV for crossing with anhydrated amorphous CsPbI 3 as the transition state. This high energy barrier also prevents the occurrence of the B→Y transition via the C-A-C pathway, consistent with experimental observations that CsPbI 3 undergoes the α → β → γ upon cooling under dry conditions . However, this does not rule out the possibility of the Y→B phase transition occurs via the C-A-C pathway under high-temperature conditions. Increasing the temperature enhances the driving force of the Y→B transition, leading to reduced critical nucleus size and lower nucleation and transition barriers (as in k B T ). Direct calculation of the energy difference for hydrated amorphous CsPbI 3 with respect to the yellow phase is unfeasible. However, it is evident that water can prompt the dissolution and degradation of CsPbI 3 , rendering the transition from the yellow phase to the amorphous structure likely to be spontaneous.