SIMPLE 2-BODY CATION-WATER INTERACTION POTENTIALS DERIVED FROM AB-INITIO CALCULATIONS - COMPARISON TO RESULTS OBTAINED WITH AN EMPIRICAL-APPROACH

Ab initio calculations were performed on M(H2O)(n) systems, M being Li +, Na+, K+, Be2+, Mg2+, or Ca2+ with n = 1, 2, 4, or 6. For the most h ydrated systems, parameters for the effective Lennard-Jones interactio n between the cation and the water molecules were determined, so as to reproduce nb initio results. In order to compare our results to those obtained previously by J. Aqvist with a purely empirical approach, wa ter-water interactions were assumed to be given by the TIP3P model. Di fferent forms for the effective two-body interaction potential were te sted. The best fits of ab initio data were obtained with a smooth r(-7 ) repulsive and a classical r(-4) attractive term, in addition to stan dard Coulombic interactions. Though better fits were obtained for alka line cations than for alkaline-earth ones, only Be2+ obviously require s a more complicated form of the potential energy function. The corres ponding parameters were tested with molecular dynamics simulations of cations in water solutions and with hydration free energy difference c alculations, using the thermodynamic perturbation approach. Radial dis tribution functions consistent with experimental data were obtained fo r all cations, Free energy differences are obviously much more challen ging. The most accurately reproduced value is the difference between t he hydration free energies of Na+ and K+, This result is likely to be significant since effective interaction energies between Na+ or K+ and water molecules as obtained in Aqvist's and in the present work are f ound to be very similar, despite the fact that the corresponding sets of parameters were determined with completely different approaches.