Inclusion of a particular trimer IJK depends upon the presence of its parents: IJ, IK, and JK. In the algorithm that we have just described, which is illustrated in Fig. , each of these parents must be present in order for IJK to be included in the PIE-DAG. We call this the M = 0 algorithm because no missing parents are permitted. Relaxing this constraint, to allow a single missing parent, affords what we call the M = 1 algorithm and this is illustrated in Fig. ). Relative to the M = 0 algorithm, use of M = 1 means considering ACD and BCD, which are thus evaluated using the low-level screening method and included in the PIE-DAG if they meet the screening criteria. Use of M = 1 may provide better accuracy in cases where one fragment significantly polarizes two others, such as an ion located between two water molecules, where the corresponding water dimer might not have been strongly interacting (due to the relatively large separation between the water molecules) when considered in the absence of the intervening ion.