The vertical and adiabatic ionization energies as well as the spatial volumes of the singly occupied molecular orbital (SOMO) of [Mg,nH(2)O](+), nless than or equal to19, were determined by ab initio calculations. Ionization energies were evaluated from Koopmans' theorem and explicitly as differences of the total energies of [Mg,nH(2)O](+) and [Mg,nH(2)O](2+) as obtained by Hartree-Fock, post-Hartree-Fock and gradient corrected density functional (DFT) methods. In the case of clusters with a sixfold coordinated magnesium cation [Mg(H2O)(6),(n-6)H2O](+) Koopmans' theorem fails for n=6-8,10. In contrast this is a valid approximation for all other cluster sizes. The most stable isomers of [Mg,nH(2)O](+), n=6-9, exhibit significantly enhanced SOMO volumes. This coincides with a significant drop in ionization energies and with an increase in electron correlation. In these clusters Koopmans' theorem is a crude approximation due to the neglect of electron correlation. The cluster size dependency of orbital relaxation and change in electron correlation upon ionization allows for an analytical fit in terms of the spatial SOMO volume. Reorganization energies and SOMO volumes indicate strong structural changes in the clusters during ionization due to a significant localization of the SOMO in [Mg,nH(2)O](+), n<6 and n>8. (C) 2003 American Institute of Physics.