A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion-water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion-water interaction potential : The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)(6)](n+), and the water molecules interact with the hydrate by means of an ab initio [M(H2O)(6)](n+)-H2O interaction potential. new ab initio interaction potential has; been developed to describe the Mn+ - (H2O)(first-shell) interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion-water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns Of simulation for a sample formed by [Cr(H2O)(6)](3+) and 512 H2O are presented. Rigidity effects, of the cluster are examined by comparing the present results with those previously obtained with a model of rigid hexahydrate [J. Phys. Chem. B 102, 3272 (1998)]. A new definition of hydrated ion based on the rotational properties of its hydrate is supported.