  Instabilities relating to cooperative octahedral tilting is common in
materials with perovskite structures, and in particular in the sub class of
halide perovskites. In this work, the energetics of octahedral tilting in the
inorganic metal halide perovskites CsPbI$_3$ and CsSnI$_3$ are investigated
using first-principles density functional theory calculations. Several low
energy paths between symmetry equivalent variants of the stable orthorhombic
(\textit{Pnma}) perovskite variant are identified and investigated. The results
are in favor of the presence of dynamic disorder in the octahedral tilting
phase transitions of inorganic halide perovskites. In particular, one specific
type of path, corresponding to an out-of-phase "tilt switch", is found to have
significantly lower energy barrier than the others, which indicates the
existence of a temperature range where the dynamic fluctuations of the
octahedra follow only this type of path. This could produce a time averaged
structure corresponding to the intermediate tetragonal (\textit{P4/mbm})
structure observed in experiments. Deficiencies of the commonly employed simple
one-dimensional "double well" potentials in describing the dynamics of the
octahedra are pointed out and discussed.
