  It was recently shown that the real part of the frequency-dependent fluidity
for several glass-forming liquids of different chemistry conforms to the
prediction of the random barrier model (RBM) devised for ac electrical
conduction in disordered solids [S. P. Bierwirth \textit{et al.}, Phys. Rev.
Lett. {\bf 119}, 248001 (2017)]. Inspired by these results we introduce a
crystallization-resistant modification of the Kob-Andersen binary Lennard-Jones
mixture for which the results of extensive graphics-processing unit (GPU)-based
molecular-dynamics simulations are presented. We find that the low-temperature
mean-square displacement is fitted well by the RBM prediction, which involves
no shape parameters. This finding highlights the challenge of explaining why a
simple model based on hopping of non-interacting particles in a fixed random
energy landscape can reproduce the complex and highly cooperative dynamics of
glass-forming liquids.
