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Our paper entitled "Pair modes of organic cations in hybrid perovskites: Insight from first-principles calculations of supercell models" is submitted to Physical Review B. It is available in arXiv ().

The disordered structures in hybrid perovskites, e.g., in the prototype material methylammonium lead triiodide (MAPbI3), are crucial to the materials properties that are closely relevant to the photovoltaic applications. Theoretical modelling of the orientational disorder of the organic methylammonium (MA) ions in MAPbI3 is very challenging, since large model systems and many samples are required to mimic the many possibilities of disordered structures. Density-functional theory (DFT), the most computationally efficient first-principles technique today, scales cubically with the system size and becomes very demanding for large MAPbI3 supercell models.

We introduce the concept of MA "pair modes", i.e., characteristic relative orientations of two neighbouring MA ions. Our DFT results of small MAPbI3 supercell models indicate that system properties, such as total energy, band gap and band structure, strongly depend on the associated pair modes. For larger supercell models with disordered MAs, our DFT calculations result in a pair-mode distribution. This distribution reveals the origin of formation of the preferential local structures in MAPbI3.

Our pair-mode concept and large-supercell-model study provide combined atomistic-statistical insight to understand the disordered structures in bulk hybrid perovskite materials. It enables our upcoming multiscale modelling of large MAPbI3 systems to study realistic hybrid perovskites under realistic conditions.