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The phonon dispersion relations of crystal lattices can often bewell-described with the harmonic approximation. However, when the potentialenergy landscape exhibits more anharmonicity, for instance, in case of a weaklybonded crystal or when the temperature is raised, the approximation fails tocapture all crystal lattice dynamics properly. Phonon-phonon scatteringmechanisms become important and limit the phonon lifetimes. We take a novelapproach and simulate the phonon dispersion of a complex dynamic solid atelevated temperatures with Machine-Learning Force Fields ofnear-first-principles accuracy. Through large-scale molecular dynamicssimulations the projected velocity autocorrelation function (PVACF) isobtained. We apply this approach to the inorganic perovskite CsPbBr$_{3}$.Imaginary modes in the harmonic picture of this perovskite are absent in thePVACF, indicating a dynamic stabilization of the crystal. The anharmonic natureof the potential makes a decoupling of the system into a weakly interactingphonon gas impossible. The phonon spectra of CsPbBr$_{3}$ show thecharacteristics of a phonon liquid. Rattling motions of the Cs$^{+}$ cationsare studied by self-correlation functions and are shown to be nearlydispersionless motions of the cations with a frequency of $\sim$0.8THz withinthe lead-bromide framework.

Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: CsPbBr$_{3}$ a phonon-liquid with Cs rattlers