  Halide perovskites are a promising class of materials for optoelectronic
applications, due to their excellent optoelectronic performance. However, they
suffer several dynamical degradation problems, the characterization of which is
challenging in experiments. Atomic scale simulations can provide valuable
insights, however, the high computational cost of traditional quantum
mechanical methods such as DFT makes it difficult to model dynamical processes
in large perovskite systems. In this work, we present a re-parameterized
GFN1-xTB method for the accurate description of structural and dynamical
properties of CsPbBr\textsubscript{3}, CsPbI\textsubscript{3}, and
CsPb(I\textsubscript{1-x}Br\textsubscript{x})\textsubscript{3}. Our molecular
dynamics simulations show that the phase stability is strongly correlated to
the displacement of ions in the perovskites. In the low temperature
orthorhombic phase, the directional movement of the Cs cations decreases
contact with the surrounding halides, initiating a transition to the
non-perovskite phase. However, this loss of contact can be compensated by
increased halide displacement, once enough thermal energy is available,
resulting in a transition to the tetragonal or cubic phases. Furthermore, we
find the mixing of halides increase halide displacement over a significant
range of temperatures, resulting in lower phase transition temperatures and
therefore improved phase stability.
