Solvation of Ca2+ in Water Studied by Born−Oppenheimer ab Initio QM/MM Dynamics

Molecular dynamics simulations based on combined ab initio molecular quantum mechanics and molecular mechanical potentials are applied to investigate the structural and dynamical properties of calcium ion solvated in liquid water. The region of highest interest, the hydration sphere of the calcium ion, is investigated by Born−Oppenheimer ab initio quantum mechanics, while the rest of the system is described by classical pair potentials. A coordination number of 8.3 is found by this high-level QM/MM method with a double-ζ basis set, in contrast to 9.2 obtained by classical pair potential simulations. Dynamical properties are related to the solvate structure. Smaller basis sets have been used to study the influence of basis sets used in the quantum mechanical part.