These sensible trends are inverted in the PBE/aDZ data. In that case, the τ 3 = 0 limit engenders catastrophic error accumulation as documented in Section 2 .1. Increasing τ 3 , which more aggressively removes subsystems from the calculation, dramatically reduces the error in ∆E int ; see Fig. . These errors are plotted as a function of the number of subsystems in Fig. , from which one sees that the first ca. 250 subsystems do reduce the error, but beyond that the additional subsystems lead to error accumulation and growth. Our interpretation is that a relatively small number of terms is required to get the gross electronic structure correct (meaning that it roughly represents the solvation environment of F - in the full cluster), but once that is achieved any fine details are washed out by cumulative SIE. For large systems, screening not only reduces the cost but also keeps error accumulation in check.