Preferential solvation of Ca2+ in aqueous ammonia solution: Classical and combined ab initio quantum mechanical/molecular mechanical molecular dynamics simulations

Abstract

Classical and combined quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been performed to investigate the solvation structure of Ca²⁺ in 18.4% aqueous ammonia solution. The classical molecular dynamics simulation has been carried out based on pairwise additive potentials. For the QM/MM scheme, the first solvation sphere of Ca²⁺ is treated by Born–Oppenheimer ab initio quantum mechanics using LANL2DZ basis sets, while the rest of the system is described based on classical pairwise additivity. The results indicate the importance of the QM treatment in obtaining a reliable geometrical arrangement as well as the correct coordination number of the solvated ion. Within the first solvation sphere of Ca²⁺, the QM/MM simulation reveals a polyhedral structure with an average coordination number of 7.2, consisting of 5.2 water and 2 ammonia molecules, compared to the corresponding value of 9.7 composed of 6.7 water and 3 ammonia molecules obtained by classical pair potential simulation. The preference for ligands is discussed on the basis of detailed simulation results.