where ν represents the molar volume of the nucleus, ∆µ represents the molar free energy difference between the black and yellow phases, and σ represents the interfacial energy of the interface between the nucleus and transitioning crystal. We use the energy difference of 26 meV/atom between the black and yellow phases of CsPbI 3 at 0 K, determined by DFT-RPA calculations, as ∆µ . This energy difference represents the maximum driving force for the B→Y phase transition in CsPbI 3 , as an increase in temperature decreases the free energy difference between the black and yellow phases. Concerning the driving force behind the reverse Y→B phase transition, free energy calculations using both classical force field and MLFF confirm that the free energy difference between the two phases-where the Gibbs free energy of the black phase is lower than that of the yellow phase-can approach the magnitude of the energy difference between the phases at 0 K as the temperature increases. Consequently, the nucleation barrier curve for the δ → α phase transition is expected to resemble that of the γ → δ phase transition.