  Strong deviations in the finite temperature atomic structure of halide
perovskites from their average geometry can have profound impacts on
optoelectronic and other device-relevant properties. Detailed mechanistic
understandings of these structural fluctuations and their consequences remain,
however, limited by the experimental and theoretical challenges involved in
characterizing strongly anharmonic vibrational characteristics and their impact
on other properties. We overcome some of these challenges by a theoretical
characterization of the vibrational interactions that occur among the atoms in
the prototypical cubic CsPbBr$_3$. Our investigation based on first-principles
molecular dynamics calculations finds that the motions of neighboring Cs-Br
atoms interlock, which appears as the most likely Cs-Br distance being
significantly shorter than what is inferred from an ideal cubic structure. This
form of dynamic Cs-Br coupling coincides with very shallow dynamic potential
wells for Br motions that occur across a locally and dynamically disordered
energy landscape. We reveal an interesting dynamic coupling mechanism among the
atoms within the nominal unit cell of cubic CsPbBr$_3$ and quantify the
important local structural fluctuations on an atomic scale.
