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

Abstract

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 +.