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For the last few years technical improvements of compositions and preparation procedures for perovskite-based solar cells have remarkably enhanced their energy conversion efficiency. Crystal and electronic structures together with ion diffusion in partially substituted cubic methylammonium (MA) perovskite based on lead halide MA0.5X0.5PbI3, where X is the cation, are theoretically investigated using density functional theory and molecular dynamics methods. The electronic structure of partially substituted perovskites studied is similar to the MAPbI3 structure, however, the band gap changes compared to their structures. Partial MA substitution distorts the lattice, which prevents MA or X diffusion between A sites in the perovskite crystal. The partial cation substitution affects the electronic structure and distorts the crystal lattice of perovskite, which makes it possible to control the optical properties of the material in different spectral regions and to influence the electronic composition of defects. [ABSTRACT FROM AUTHOR]