ON THE EFFECTS OF TRUNCATING THE ELECTROSTATIC INTERACTIONS - FREE-ENERGIES OF ION HYDRATION

Free energies of ion hydration have been evaluated by using the thermo dynamic integration technique (the slow growth method) in molecular dy namics simulations. Ionization processes of Na, Ca, and Cl have been c onsidered. The cutoff method and the Ewald summation approach have bee n used alternatively in truncating the long-range electrostatic intera ctions. The extended simple point charge (SPC/E) model [J. Phys. Chem. 91, 6269 (1987)] and the ion-water potential of Straatsma and Berends en [J. Chem. Phys. 89, 5876 (1988)] have been used in out simulations. The results have been tested against experimental measurements and th e Ewald method has led to the best performances. A good agreement with the Born model was found in the charging process of a calcium ion. Ch anges in the organization of water molecules around the ion have been evaluated from equilibrium molecular dynamics simulations performed at different stages of this process. Ion-water radial distributions func tions, and the orientation of water molecules as well as residence tim es have been analyzed. (C) 1996 American Institute of Physics.