In each case, we investigated four low-energy isomers (see Figs. and) with structures taken from Refs. 77 and 76, respectively. These ion-water clusters are more challenging for the MBEs, since larger higher-order contributions can be expected due to the polarizability of the water molecules. Previously, we investigated the accuracy of the relative energies of these isomers of (H those for the neutral water hexamers (see Fig. ), but the errors scatter more widely, i.e., the errors in the relative energies will be substantially larger.