We will refer to the two-body expansion described above as H-MFCC-MBE(2)-ff. To decrease the computational cost, it is possible to introduce a cutoff λ and to neglect fragment-fragment interactions ∆E ff ij if the distance between two fragments (defined as the shortest distance between any two atoms) is above this threshold. Introducing such a threshold ensures that the scaling of the computational effort with the number of fragments remains linear.