With the db-MBE(2), the errors are reduced to a maximum of 10.0 kJ/mol and 8.3 kJ/mol for (H 3 O) + (H 2 O) 5 and (OH) -(H 2 O) 5 , respectively. This, roughly twice as large as for the neutral hexamers, demonstrates the increased complexity of the charged clusters and the larger importance of higher-order contributions. As shown in Ref. for ion-water clusters, the db-MBE greatly benefits from the use of embedding schemes in the fragment calculations, which improves the convergence of the electron densities and substantially lowers the error of the db-MBE(2).