Anharmonic lattice dynamics in large thermodynamic ensembles with machine-learning force fields: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Cs</mml:mi><mml:mi>Pb</mml:mi><mml:msub><mml:mi>Br</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> , a phonon liquid with Cs rattlers

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