Structural and electronic properties of metal halide perovskites using non-periodic calculations: a combined xTB,...

We present a computational protocol to study the structural and optoelectronic properties of metal halide perovskites using non-periodic calculations (i.e., a cluster-based approach). The proposed framework strategically combines xTB, DFT and ONIOM methods, leveraging their strengths for accurate results with reduced computational cost. Using the CH₃NH₃PbI₃ perovskite (MAPI) as a reference system, several cluster models (ranging from 204 to 2175 atoms) were constructed to preserve the chemical environment of the bulk phase and maintain electroneutrality. Structures were optimized with the GFN2-xTB method, while optoelectronic properties were computed at the PBE0-D4/Def2-TZVP level. For the larger systems (>1000 atoms), a two-layer ONIOM scheme was applied, treating the region of interest at the PBE0-D4/Def2-TZVP level, with the rest of the system treated using GFN2-xTB with electrostatic embedding. Our results show that GFN2-xTB is suitable for studying the structural properties of perovskites. Regarding optoelectronic properties, the DFT-calculated bandgap is strongly overestimated for smaller clusters due to nanosizing effects. Meanwhile, for larger systems, results based on a two-layer ONIOM approach significantly enhance the accuracy of computed optoelectronic properties. Overall, this framework offers a reliable and efficient alternative to periodic calculations, broadening the range of computational tools available for studying halide perovskites.