Experimental observations of the B↔Y phase transitions in CsPbI 3 typically employ nanowires as material models . We constructed three different CsPbI 3 nanowire models to understand the water-MHP interactions:the black phase (Fig. ), the yellow phase (Fig. ), and a half-black and half-yellow (B-Y) structure (Fig. ). For the B-Y nanowire, the (100) face of the black phase contacts the (100) face of the yellow phase, as this B/Y interface has been confirmed experimentally . 1500 water molecules were randomly placed near the nanowire surface in these three systems. MD simulations were performed at 300 K (Figs. 2a-c left panels) and 600 K (Figs. right panels) and 1 atm for 1 µs. The simulation results show that water molecules do not penetrate the nanowires with pure yellow or black phases (Figs. ). They form a thin water layer on the surfaces at low temperatures or mostly water vapor at high temperatures, with only a few water molecules adsorbed on the nanowire surface. This observation agrees with in situ ambient pressure X-ray photoemission spectroscopy experiments on CsPbI 3 . Since air molecules were not considered in our nanowire system, the water content cannot be directly correlated with the relative humidity in experimental conditions. However, the water content used in our simulations is appropriate. An insufficient amount of water molecules cannot effectively accelerate the phase transition, whereas an excessive amount can lead to the dissolution of CsI, prompting the transformation from CsPbI 3 to CsPb 2 I 5 and eventually to PbI 2 . The water content in our simulated system is sufficient to effectively accelerate the B→Y phase transition of CsPbI Fig. ).