In order to obtain a robust PES, the fragmentation scheme must take account of the large range of configurations shown in Figure . A key aspect is the proximity of the two groups of three H-atoms on each methyl rotor at each end of C 14 H 30 . So while the intermolecular Morse variables are very small for these two groups in the linear chain, they are not as small for many configurations. Thus, at least one fragment should include these six H atoms.