  The liquid-liquid phase transition in high-pressure Hydrogen is a problem of
longstanding and controversy. The recent Nature paper by Cheng et al. [vol.
585, p. 217] makes a set of strong claims to the effect that all the previous
density functional theory molecular dynamics (MD-DFT) and quantum Monte Carlo
calculations of that transition are incorrect because of finite size effects
and, in the MD-DFT case, short run times. The basis of those claims is their
use of large systems and long durations for classical MD driven by a
machine-learnt potential (MLP) which they developed. The straightforward test
of their claims is to do MD-DFT on systems as large or larger than Cheng et al.
used and for significantly longer durations than in the previous MD-DFT
simulations. We have done so and find that neither diagnosis of theirs (size
effects, duration limits) is correct. Instead, we find that the MLP does not
drive MD in fidelity with the underlying DFT electronic structure that it is
supposed to replicate. The result is that the MLP-driven MD results are
artifactual, not systematically connected to the theoretical underpinning on
which the MLP was trained.
