where the threshold τ 3 ranges from τ 3 = 0 (no screening) to τ 3 = 0.4 kcal/mol (aggressive screening). In building the aforementioned graph representation of the nbody interactions, a new fragment is added to scheme S (x) if its constituent lower-order terms ("parents") are present in S (x) . For example, F ABC is added only if F AB ∈ S (x) , F AC ∈ S (x) , and F BC ∈ S (x) . An additional parameter M allows for addition of subsystems that are missing at most M of their parents. For example, if M = 1 then F ABC is added if two of its three parents {F AB , F AC , F BC } are present in S (x) .