AB , with a similar meaning for E F B ∩i . This is operationally equivalent to dropping all terms in eqn. (1) that do not contain nuclei from either F A or F B . Precomputing coefficients for eqn. (7) leads to a substantial reduction in the number of subsystem calculations, simply by avoiding subsystems where C i,x = 0. The number subsystem calculations required to apply MBE(3) to F -(H 2 O) N clusters ranging up to N = 25 is plotted in Fig. . For N = 25, a priori calculation of the C i,x coefficients eliminates 78% of the possible subsystems.