The original training set included 3000 configurations (1000 configurations at each of the three temperatures), while for the test we used 2000 configurations collected from the MD simulation performed at 300 K. The representative configurations for equation (1) were generated as follows. We picked 250 configurations from the original small basis set MD run, and for each of these, we displaced each of the atoms, in turn, by 0.5 Å along each Cartesian direction. This results in M = 27 × 3 × 250 = 20250 configurations and corresponding basis functions. Note that we do not need QM energies or forces for these configurations, since they do not enter the fit, just serve to generate basis functions. We found that this procedure worked significantly better than just picking all representative configurations randomly from the MD trajectory itself.