In that work the "monomers" are CH 3 and CH 2 and the many-body series truncates at the 4-b terms, i.e., fragments with at most 4 carbon atoms. Each n-body term is fit using PIPs, as above. (This representation is augmented by long-range polarization terms, but this is not relevant to the present work.) The representation was applied to several hydrocarbons up to decamer, with encouraging results, albeit without extensive investigation of the final PES. This important paper was a "proof of principle" of the power of this approach. It was noted that the basic idea of using a many-body approach to fit a molecular potential had been suggested previously using fragmentation ideas. Unified Fitting