Adiabatic Approximation in Explicit Solvent Models of RedOx Chemistry.

We propose a calculation scheme that accelerates QM/MM simulations of solvated systems. This new approach is based on the adiabatic approximation whereby the solute degrees of freedom are separated from those of the solvent. More specifically, we assume that the solute electron density remains constant with respect to the relaxation of the solvent molecules. This allows us to achieve a dramatic speed-up of QM/MM calculations by discarding the slow self-consistent field cycle. We test this method by applying it to the calculation of the redox potential of aqueous transition metal ions. The root-mean-square deviation (RMSD) between the full solvation and adiabatic approximation is only 0.17 V. We find a RMSD from experimental values of 0.32 V for the adiabatic approximation as compared to 0.31 V for the full solvation model, so that the two methods are of essentially the same accuracy. Meanwhile, the adiabatic calculations are up to 10 times faster than the full solvation calculations, meaning that the method proposed here reduces the cost of QM/MM calculations while retaining the accuracy.