The librational and vibrational motions of water molecules in the first and second hydration spheres of the Ni2+ ion were evaluated by means of velocity autocorrelation functions obtained by classical and combined quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations. The rotational frequencies around three principal axes and the intramolecular vibrational frequencies of the water molecule were calculated using normal-coordinate analyses. The rate constant of the water exchange in the second hydration sphere of the Ni2+ ion was estimated to be ca. 5 x 10(10) s(-1). A water-exchange mechanism with an associative character appears to be predominant, but less associative exchanges are also observed. (C) 2002 Elsevier Science B.V. All rights reserved.