The structural arrangement and dynamics of solvated Mg2+ in dilute aqueous solution have been studied by ab initio QM/MM molecular dynamics simulation, in which the whole first and most of the second hydration shell of the ion were treated at Hartree-Fock level using LANL2DZ basis sets. Besides the most stable Mg2+(H2O)(6) species, intermediates such as Mg2+(H2O)(5)(H2O), where at least one water molecule temporarily moves into the inter-shell region but remains H-bonded to inner-shell water, as well as transition complexes of the Mg2+ (H2O)(6)(H2O) type, exist in aqueous solution. The dynamics of solvate and surrounding water molecules are discussed in connection to the 'structure-forming' ability of Mg2+. (c) 2005 Elsevier B.V. All rights reserved.