This basis is the union of 2-b, 3-b and 4-b bases, each with different Morse range parameters and distance cutoffs. Rather than optimizing these hyperparameters, they were set according to the general expectation that the n-body interactions fall off as n increases. For the 2-body, CH, HH, and CC Morse variables, a single range parameter of 2.5 bohr was used. Also a maximum power of 10 is used, so the total number of 2-body coefficients is 30. The cutoff distance for 2-body is 10 Å, that is, if the distance between two atoms is larger than 10 Å, that pair does not contribute to the total energy. Between 9 Å and 10 Å, a polynomial switching function is applied that damps that interaction to zero. For the 3-body, the Morse range parameter is 1.8 bohr, and the maximum polynomial order of the PIPs is 8.