The PIP basis we used for 21-aspirin is of permutational symmetry 322221111111111; this indicates that the basis functions are invariant with respect to permutations of three atoms (the methyl H atoms) as well as four groups of two atoms and that the remaining 10 atoms are treated as distinguishable. Two PIP bases were used to fit. One consists of 25,001 and the other 49,978 polynomials and coefficients. These were was "pruned" from the basis of 297,952 (the basis for for a maximum total polynomial of 3). Some details of the performance of these two PIP PESs (denoted PES3 and PES4) compared to other ML methods are given below.