A short reminder that PIPs are invariant with respect to permutations of like atoms, a fundamental property of potentials. A simple example that illustrates this is for the H 2 CO molecule, which is the smallest non-trivial symmetric group of order 2. We denote this class 4-atom molecules by A2BC and in more abbreviated notation by 211. The potential is invariant with respect to the interachange of the two H atoms; in general the two A atoms. An expression for the potential that respects this symmetry using the six internuclear distances is given by