Born-Oppenheimer ab initio QM/MM dynamics simulations of Na+ and K+ in water:: From structure making to structure breaking effects

The description of nonadditive contributions in the first hydration shell of Na+ and K+ has been improved by performing molecular dynamics simulations based on combined ab initio quantum mechanical and molecular mechanical potentials. The active-site region, the first hydration sphere of ions, is treated by Born-Oppenheimer ab initio quantum mechanics, while the environment is described by classical pair potentials. The average coordination numbers obtained by this high accuracy method, with valence double-zeta basis sets for water and Los Alamos ECP plus DZ basis sets for cations, lead to a lower value of 5.6 +/- 0.3 for Na+ and a higher value of 8.3 +/- 0.3 for K+, respectively, compared to the corresponding values of 6.5 +/- 0.2 and 7.8 +/- 0.2 resulting from pair potentials. The effects of nonadditive terms are also found to play a significant role in the preferential orientation of water molecules within the first hydration shell of Na+ and K+. The experimentally observed "structure-breaking" effects of K+ are well reflected and explained on a molecular basis by the simulation results.