In atomic perovskites, the tolerance factor has been successfully used not only to suggest whether a composition will be a perovskite, but also to provide a first indication of the magnitude of octahedral tilts, e.g. CaTiO 3 adopts the distorted a -a -c + tilt system at room temperature, whereas SrTiO 3 is cubic with no tilts. Although the tolerance factor approach explains which molecular AMX 3 frameworks are likely to crystallise with a perovskite structure, it does not account for either the magnitudes or kinds of framework distortions observed. For example, in the series of formate perovskites A{Mn(HCO 2 ) 3 } where A + = Rb + 48 , CH 3 NH 3 , (CH 3 ) 2 NH 2 and (CH 2 ) 3 NH 51 (arranged in increasing size of A + /increasing τ), there is no systematic trend in the size or pattern of the octahedral tilting, respectively: