  The phonon dispersion relations of crystal lattices can often be
well-described with the harmonic approximation. However, when the potential
energy landscape exhibits more anharmonicity, for instance, in case of a weakly
bonded crystal or when the temperature is raised, the approximation fails to
capture all crystal lattice dynamics properly. Phonon-phonon scattering
mechanisms become important and limit the phonon lifetimes. We take a novel
approach and simulate the phonon dispersion of a complex dynamic solid at
elevated temperatures with Machine-Learning Force Fields of
near-first-principles accuracy. Through large-scale molecular dynamics
simulations the projected velocity autocorrelation function (PVACF) is
obtained. We apply this approach to the inorganic perovskite CsPbBr$_{3}$.
Imaginary modes in the harmonic picture of this perovskite are absent in the
PVACF, indicating a dynamic stabilization of the crystal. The anharmonic nature
of the potential makes a decoupling of the system into a weakly interacting
phonon gas impossible. The phonon spectra of CsPbBr$_{3}$ show the
characteristics of a phonon liquid. Rattling motions of the Cs$^{+}$ cations
are studied by self-correlation functions and are shown to be nearly
dispersionless motions of the cations with a frequency of $\sim$0.8THz within
the lead-bromide framework.
