In experiments, CsPbI 3 nanowires typically have diameters greater than 100 nm and exhibit properties more akin to bulk materials. Therefore, we also constructed a B-Y bulk material model with three-dimensional periodic boundary conditions (supplementary material, computational Methods and Fig. ). The main advantages of using the bulk material model in simulations are the elimination of the surface effects on phase transitions and the convenience of sampling. In the bulk models, 0, 50, 100, and 150 water molecules were placed at the B/Y interfaces for each system (0, 25, 50, and 75 water molecules per interface), and MD simulations were performed for each system at 700 K for 1.5 µs. Similar to the nanowire simulations, the Y→B transition rate in the B-Y bulk systems also shows sensitive dependence on the water content. For example, while the zero-water system shows minimal change, the 150-water system shows nearly a complete Y→B phase transition in 320 ns (supplementary material, Fig. ). During the phase transition, we observed that water molecules primarily accumulate at the interface, where they can move along the interface and undergo overall translation as the interface propagates. However, these water molecules rarely penetrate the crystalline region (supplementary material, Fig. ). Fig. shows the time evolution of the fraction of the black phase with different water contents at 700 K. Here, the water content is given in water areal density, ρ = n W /2S, where n W is the number of water molecules, and S is the area of y-z plane of the simulation box. Fig. shows v as a function of ρ at different temperatures. For a particular simulated temperature, the transition rate increased nonlinearly with the number of water molecules (Figs. 3a-b and supplementary material, Fig. ).