We present a simple interacting tight-binding model for excitons, which is used to investigate the dependence of the exciton binding energy of CH 3 NH 3 PbI 3 over the disorder induced by the molecular motion at room temperature. The model is fitted to the electronic structure of CH 3 NH 3 PbI 3 by using data from density-functional theory and Born-Oppenheimer ab initio molecular dynamics, and it is solved in the mean-field approximation. When a finite-scale analysis is performed to extract the

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... energetic of the excitons at experimental concentrations we find that disorder in general reduces the binding energy of about 10%. This suggests that the excitonic properties of CH 3 NH 3 PbI 3 largely depend on the electronic structure of the PbI 3 inorganic lattice.