Ion-water clusters present a particularly challenging test case for the MBE . There have been numerous computational studies of ion-water clusters that employ a MBE (for recent examples, see Refs. . Because of the large polarization effects, three-body and four-body effects generally play an important role and the conventional MBE cannot be truncated at low order. To some degree, this can be alleviated with more general fragmentation schemes based on overlapping fragments .