Title: Dynamic Local Symmetry Fluctuations of Electron Density in Halide Perovskites

Abstract: Metal halide perovskites have emerged as an exciting class of materials for applications in solar energy harvesting, optical devices, catalysis, and other photophysical applications. Many of the exciting properties of halide perovskites are tied to their soft, dynamic, and anharmonic lattice. In particular, the precise coupling between anharmonic lattice dynamics and electronic fluctuations is not completely understood. To build an understanding of this coupling, we use ab initio molecular dynamics simulations supplemented by the calculation of maximally localized Wannier functions to carry out a dynamic group theory analysis of local electron density fluctuations and how these fluctuations are coupled to lattice fluctuations in the model inorganic halide perovskite CsSnBr3. We detail symmetry-dependent couplings between vibrational modes, including octahedral tilting. Importantly, we suggest that the large anharmonicity of some of the vibrational modes in CsSnBr3 result from electron rotation--nuclear translation coupling, in analogy to rotation--translation coupling effects in molecular plastic crystals. We also identify electronic fluctuations in the Cs cation that couple to distortions in the surrounding Sn-Br cubic coordination environment. We anticipate that our approach and resulting insights into electronic fluctuations will aid in further understanding the role of the fluctuating lattice in determining important physical properties of halide perovskites and beyond.