An important question about electron-transfer proteins is how the environment of the redox site created by the protein’s matrix affects their electron-transfer properties. Here, we investigate intramolecular electron transfer in the [4Fe-4S] ferredoxins, which are a class of iron-sulfur electron-transfer proteins found in numerous electron transport chains, including the photosynthetic pathway. These proteins are characterized by having two [4Fe-4S] clusters, often but not always with the same reduction potential, and by the pseudo2-fold symmetry of the protein backbone. The nuclear polarization is calculated from molecular dynamics simulations of Clostridrium acidiurici ferredoxin, with a total of 6 ns of simulation, and is then used to calculate free energy reaction curves. In addition, we present here a new method, referred to as the Gaussian parabola method, for obtaining the reaction energy ¢G° and the reorganization energy I from the mean and fluctuations of the polarization, which is based on the linear response of a system with Gaussian fluctuations. For ferredoxin, the calculated outer sphere I is small (<200 meV) and is consistent with the lack of temperature dependence in experimental measurements of rates for this protein; however, the rate calculated from the calculated I is consistent with experimental values if the inner sphere I is large. The calculations also indicate that the contribution of the protein to I is smaller than that of the solvent, which implies that the protein enhances the rate of electron transfer by providing an environment that has a low reorganization energy.