A Polarizable QM/MM Explicit Solvent Model for Computational Electrochemistry in Water

We present a quantum mechanical/molecular mechanical (QM/MM) explicit solvent model for the computation of standard reduction potentials E-0. The QM/MM model uses density functional theory (DFT) to model the solute and a polarizable molecular mechanics (MM) force field to describe the solvent. The linear response approximation is applied to estimate E-0 from the thermally averaged electron attachment/detachment energies computed in the oxidized and reduced states. Using the QM/MM model, we calculated one-electron E-0 values for several aqueous transition-metal complexes and found substantially improved agreement with experiment compared to values obtained from implicit solvent models. A detailed breakdown of the physical effects in the QM/MM model indicates that hydrogen-bonding effects are mainly responsible for the differences in computed values of E-0 between the QM/MM and implicit models. Our results highlight the importance of including solute-solvent hydrogen-bonding effects in the theoretical modeling of redox processes.