A combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to evaluate the structural and dynamical properties of Co(III) in water. The average Co(III)-O distance resulting from the simulation is 1.97 Angstrom for the first hydration shell, and the coordination numbers are 6/15 for first/second hydration shell. Ligand exchanges between the second hydration shell and the bulk occur with a mean ligand residence time of similar to55 ps, estimated from three events observed during the simulation. The hydration enthalpy of -1144+/-8 kcal/mol, determined by the QM/MM molecular dynamics simulation, is in good agreement with experimental estimations. (C) 2003 American Institute of Physics.