ating ∆E low IJKL••• . This natural attenuation is demonstrated in Fig. using (H 2 O) 20 clusters. For each cluster, we ran the bottom-up algorithm up to n = 20, with τ 3 = 0.05 kcal/mol and M = 1. For 10 out of the 12 clusters in the data set, we found that the procedure terminated naturally (due to a lack of parents) at n = 7, and for the remaining two clusters it terminated at n 9.