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Two seemingly similar crystal structures of the low-temperature (~100 K)MAPbX$_3$ (X=I,Br,Cl) perovskites, but with different relative Methylammonium(MA) ordering, have appeared as representatives of this orthorhombic phase.Distinguishing them by X-ray diffraction experiments is difficult andconventional first-principles based molecular-dynamics approaches are often toocomputationally intensive to be feasible. Therefore, to determine thethermodynamically stable structure, we use a recently introduced on-the-flyMachine-Learning Force Field method, which reduces the computation time fromyears to days. The molecules exhibit a large degree of anharmonic motiondepending on temperature: i.e. rattling, twisting and tumbling. We observe thecrystal's 'librational pathways' while slowly heating it in isothermal-isobaricsimulations. Marked differences in the thermal evolution of structuralparameters allow us to determine the real structure of the system via acomparison with experimentally determined crystal structures.

Exploring librational pathways with on-the-fly machine-learning force fields: Methylammonium molecules in MAPbX$_3$ (X=I, Br, Cl) perovskites