COMPUTER-SIMULATION STUDY OF THE MEAN FORCES BETWEEN FERROUS AND FERRIC IONS IN WATER

With molecular dynamics and Monte Carlo trajectories, we have studied the potential of mean force between two iron ions in a periodically replicated simple point charge (SPC) model of liquid water. The results of these calculations are similar whether the primary cell contains 430 or 1022 water molecules. In contrast, the results depend significantly upon the treatment of the long-ranged forces. Specifically, if the long-ranged forces are truncated at half the box length (either discontinuously or with smooth splines), the potential mean force between Fe2+ and Fe3+ ions is nonmonotonic, exhibiting a powerful attraction at interionic distances, R, beyond 6 angstrom. If, however, Ewald sums are performed, the potential is monotonic and close to zz'/epsilon-R. Here, z and z' refer to the ionic charges, and epsilon almost-equal-to 70 is the dielectric constant of SPC water.