Classical and QM/MM MD simulations of sodium(I) and potassium(I) ions in aqueous solution

The optimized energy and geometry for Na+- and K+-water complexes in gaseous phase have been studied using different methods such as HF, MP2, B3LYP, and CCSD. Classical and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations have been performed to describe structural and dynamical properties of Na+ and K+ in water and to compare the two approaches. The first hydration shell was treated by ab initio quantum mechanics at restricted Hartree-Fock (RHF) level using the Los Alamos LANL2DZ ECP basis set for the cations Na+ and K+ and Dunning double-ζ plus polarisation basis set for O and H. The results obtained are in good agreement with previously published experimental and theoretical results. In the QM/MM method, the radial distribution function (RDF) showed the maximum probability of the Na-O and K-O bond lengths at 2.39 Å and 2.85 Å, respectively. A considerable number of water exchange reactions were observed within the first shell during the simulation time of 16 and 12 ps of Na+ and K+ ions in aqueous solution, respectively. The number of exchange events in both first and second shells is higher in the case of K+ than Na+ indicating the weaker ion-ligand strength of K+.