A new nonempirical method of determining the effective pair potential functions which are suitable for the molecular simulation of heterolysis reactions has been proposed. The self-energy correction due to the polarization in solution was estimated by the ab initio GB calculation which includes the solvent effect by the continuum model using the generalized Born formula, and the polarization caused by solvation was incorporated in the effective pair potential functions. The method was applied to the ionic Fragmentation reaction of t-BuCl in aqueous solution. The effective pair potential functions between t-BuCl and water were determined at 12 different C-Cl distances, and expressed by analytical functions which cover the whole reaction stage, from the covalent bonding region to the dissociated free ions. The Monte Carlo simulation and statistical perturbation theory using the effective pair potential functions determined the free energy profile of the reaction with a reasonable dissociation energy.