Two separate databases were used to construct the ∆V energies. The two data sets were fit separately using 4th-order 222111-symmetry PIPs which are functions of Morse variables with a range parameter of 2.5 bohr. Further symmetrization was done, as described in detail in Ref. 10, so that the fit is also invariant with respect to all permutations of the three monomers. The fitting RMS error is 9 cm -1 for the first data set and 2 cm -1 for the second one. The two fits were smoothly connected through a switching function when the maximum OO distance is in [5.0, 5.5] Å. Finally, for the consideration of both computationally efficiency and accuracy, we applied an additional smooth switching function when the maximum OO distance is within [6.0, 7.0] Å for the ∆V 3-b correction potential to make it decay to 0 smoothly in the long-range region.