that is a linear combination of subsystem energies E i , with coefficients C i,x derived from previous state (x -1) using the inclusion/exclusion principle. To calculate ∆E int for a system F AB , we define two subsystems F A = {A 1 , A 2 , . . .} and F B = {B 1 , B 2 , . . .} such that F AB = F A ∪ F B and F A ∩ F B = ∅. The interaction energy for any fragmentation scheme S (x) AB corresponding to F AB can be computed as