prior DFT-based studies [@Tambe2008BulkStudy; @Sjostrom2016MultiphaseTheory], and experiment [@Fiquet2019StructuralGPa], the solid-to-solid transition out of FCC should require hundreds of GPa.

At higher pressures, BCC becomes the preferred crystal phase. Figure \[fig:melt\_curve\] shows the liquid-BCC coexistence curve pressures around [${250}\,\mathrm{GPa}$]{}. The ANI-Al predictions are within 5% of prior DFT calculations [@Sjostrom2016MultiphaseTheory]. Interestingly, in performing these simulations, the ANI-Al potential is extrapolating well beyond its training data. During active learning, we collected DFT training data only over a limited region of atomic densities (yellow region of Fig. \[fig:coldcurveAl\]a) reaching up to about [${60}\,\mathrm{GPa}$]{} (Fig. S1 of the SI).

Phase transition dynamics {#results:phasechange}
-------------------------

![image](images/melt-freeze_MD.pdf){width="100.00000%"}

Next we carry out a nonequilibrium MD simulation to observe both freezing and melting dynamics. Our intent is to validate the ANI-Al predicted energies and forces at snapshots along the dynamical trajectory. Along the trajectory the temperature is slowly increased from [${300}\,\mathrm{K}$]{} to [${1500}\,\mathrm{K}$]{}, then cooled back to [${300}\,\mathrm{K}$]{}. The details of these simulations are provided in Section 1.4.2 of the supplemental information.

Figure \[fig:almeltfreeze\] shows the potential energy, mean force magnitude, and pressure for both ANI-Al and DFT along this trajectory. Melting from FCC to liquid occurs at around [${300}\,\mathrm{ps}$]{} and freezing occurs around [${700}\,\mathrm{ps}$]{}. Pressure was calculated 