To assess the accuracy of the db-MBE for ion-water clusters, we will consider different test cases in the following. First, we investigate the absolute error in the total interaction energies of Ca 2+ -water clusters of increasing size (see Section 3.1). Second, we evaluate the accuracy of the relative energies of different structural isomers of selected F --water clusters (see Section 3.2), OH --water clusters (see Section 3.3), and H 3 O + -water clusters (see Section 3.4). In each case, the ion (Ca 2+ , F -, OH -, or H 3 O + ) is treated as a separate, charged fragment in the MBE. For the H 3 O + -water clusters, the additional proton is assigned to the closest water molecule to form the H 3 O + ion.