Effect of solvent polarization on the reorganization energy of electron transfer from molecular dynamics simulations

The solvent contribution λs to the reorganization energy of electron transfer can be estimated from averages of the potential energy gaps between neutral-pair and ion-pair states over an ensemble of structures generated from molecular dynamics simulations. Invoking a Marcus-type two-sphere model for charge separation and recombination in an aqueous environment, we explored the effect of a polarizable force field and noted a strong reduction of λs (by ∼45%) compared to the corresponding value obtained with a standard nonpolarizable force field. Both types of force fields yield λs values that in agreement with the Marcus theory, vary strictly linearly with the inverse of the donor-acceptor distance; the corresponding slopes translate into appropriate effective optical dielectric constants, ε∞ ≈1.0±0.2 for a nonpolarizable and ε∞ ≈1.7±0.4 for a polarizable force field. The reduction in the solvent reorganization energy due to a polarizable force field translates into a scaling factor that is essentially independent of the donor-acceptor distance. The corresponding effective optical dielectric constant, ε∞ ≈1.80, is in excellent agreement with experiment for water.