• is an n-mer interaction contribution. For explicit expressions for the n-mer interaction contributions we refer to, e.g., Ref. . The above expansion is truncated at order n. For such a truncated MBE, the number of subsystem calculations scales as O(N n ). Usually, one aims at truncating the MBE at low order (ideally, already at second order), while the exact, supermolecular result is recovered by construction for n = N .