SIMULATION OF WATER SOLUTIONS OF NI2+ AT INFINITE DILUTION

A new ab initio pair potential is developed to describe the nickel-wat er interactions in Ni(II) aqueous solutions. Results of Monte Carlo si mulations for the Ni(II)-(H2O)200 system are presented for this pair p otential with and without three-body classical polarization terms (the water-water interaction is described by the ab initio MCY potential). The structure of the solution around Ni(II) is discussed in terms of radial distribution functions, coordination numbers and thermal ellips oids. The results show that the three-body terms have a non-negligible effect on the simulated solution. In fact, the experimental coordinat ion number of six is reproduced with the full potential while a higher value is predicted when the simple pairwise-additive potential is use d. The equilibrium NiO distance for the first hydration shell is also dependent on the use of the three-body terms. Comparison of our distri bution functions with those obtained by neutron-diffraction experiment s shows a reasonable quantitative agreement. Statistical pattern recog nition analysis has also been applied to our simulations in order to b etter understand the local thermal motion of the water molecules aroun d the metal ion. In this way, thermal ellipsoids have been computed (a nd graphically displayed) for each atom of the water molecules belongi ng to the Ni(II) first hydration shell. This analysis revealed that th e twisting and bending motions are greater than the radial motion, and that the hydrogens have a higher mobility than the oxygens. In additi on, a thermodynamic perturbation method has been incorporated in our M onte Carlo procedure in order to compute the free energy of hydration for the Ni(II) ion. Agreement between these results and the experiment al ones is also sufficiently reasonable to demonstrate the feasibility of this new potential for the nickel-water interactions.