Molecular simulations have been widely used to study the role of water molecules in the B↔Y phase transition of MHPs . DFT calculations show that the addition of a water molecule to a 20-atom CsPbI 3 system reduces the energy barrier of the B↔Y phase transition by 10 meV/atom through the one-step pathway . However, the direct penetration of water molecules from the crystal surface into the crystal interior requires overcoming a larger energy barrier of 0.46 eV, making it less likely . On the other hand, the energy barrier for water molecules to enter the perovskite material from grain boundaries is much smaller . MD simulations of the water/MAPbI 3 and water/CsPbI 3 two-phase systems suggest that surface I ions can dissolve into the water layer, resulting in the creation of iodine vacancies . Consequently, it has been suggested that these vacancies could decrease the energy barrier for the B↔Y phase transition . Yet, prior computational studies have been constrained by the spatial and temporal scale of the examined systems, indirectly inferring the mechanism of the water-assisted B↔Y phase transition from simulation outcomes of small-scale systems. Consequently, simulations for large systems with long dynamics are necessary to replicate these phase transitions and elucidate the microscopic mechanism of the water-assisted B↔Y phase transition.