Combined quantum mechanics/molecular mechanics-molecular dynamics simulations (QM/MM-MD) were carried out at Hartree-Fock (HF) and density functional theory (DFT) level for Ca2+ in water to investigate the exchange process of water molecules between first and second hydration shell. In both cases one exchange reaction took place during a simulation time of about 10 ps. The time needed for this water exchange is 1.2 ps in the DFT/MM and 2.0 ps in the QM/MM-HF simulation. From the data obtained so far, the exchange rate can be estimated to be larger than 10(-11) s.