Density functional theory calculation has been performed to calculate the redox potential and the correct ground spin state of iron complexes in acetonitrile. Widely used B3LYP functional is applied with the spin state corrected basis sets. The newly developed protocol for the set of 21 iron complexes is to optimize the structure at the level of the B3LYP/6-31G* and to calculate the single point electronic energy with the same functional and the modified basis sets s6-31G* for the iron atom and 6-31+G* for other ligand atoms. The solvation energy is considered through the polarized continuum model and the cavity creation energy is included for the accurate spin state description. Modifying the cavity size by employing the different scaling factor according to the mean absolute value of the natural population analysis charge (MA-NPA) is introduced. The molecule with the large MA-NPA requires the cavity size smaller than the less polar one. This protocol gives only 1 wrong ground spin state among the 18 iron complexes for which experimental data are known. For the open circuit voltage (OCV) calculation, our protocol performs well yielding the mean absolute error of 0.112 V for the test set. The close correlation between the calculated and the experimental OCV are obtained. © 2013 Wiley Periodicals, Inc.