in which N is the number of selected atoms, x i (t) is the coordinates of atom i at time t, x ref i is the coordinates of reference atoms, and w i is the weight whose default value is 1. Timedependent RMSD (Figure ) for halide atoms (Br and I) in 2D monolayer PEA 2 SnBr 4 and PEA 2 SnI 4 oscillate within a narrow range compared to the DFT-optimized structure.