Computing Standard Reduction Potentials of Solvated Species: Accuracy of VASPsol and VASPsol++ versus SMD and COSMO-RS Solvation Methods

Standard reduction potentials (E⁰) describe the tendency of a species to be reduced or oxidized, playing an important role in determining electron transfer thermodynamics and kinetics. Different solvation models are available for the computational prediction of E⁰ values, such as SMD/COSMO-RS for atom-centered DFT codes versus VASPsol/VASPsol++ for plane-wave DFT codes. In this study, we evaluate the performance of relatively new solvation models implemented in plane-wave DFT codes (VASPsol and VASPsol++), as compared to more established methods often implemented in atom-centered DFT codes (SMD and COSMO-RS). We compute the standard reduction potentials for a set of 56 chemically diverse organic molecules in acetonitrile. The atom-centered methods show mean absolute errors (MAEs) ranging from 0.19 to 0.36 V. VASPsol delivered comparable performance to the majority of atom-centered methods (MAE = 0.37-0.39 V). VASPsol++ achieved improved accuracy with MAE values between 0.18 and 0.19 V. We also tested the accuracy of VASPsol and VASPsol++ for ten additional organic molecules in protic media (H₂O), achieving MAE values of 0.19 and 0.12 V, respectively. These results highlight that a unified plane-wave DFT approach, employing VASPsol or the enhanced VASPsol++ protocol, can be used to calculate both standard reduction potentials of solvated species, as well as band edge alignments at aqueous-solid interfaces.