(a) Potential energy as a function of tilt amplitude for in-phase and out-of-phase tilts in CsPbI3 and CsSnI3. Circles and squares in all four panels represent fixed and tetragonally relaxed unit cells, respectively. (b) The 2D octahedral tilting PES in CsPbI3 with the out-of-phase tilt around the pseudo-cubic axis fixed to its value in the fully relaxed a−a−c+ structure. The x coordinate axis gives the magnitude of the in-phase tilt around the pseudo-cubic c axis, while the y coordinate axis gives the magnitude of the out-of-phase tilt around b pseudo-cubic axes. From Klarbring (2019). Reproduced with kind permission from the American Physical Society.