The hydration structure of the lithium ion

The hydration structure of Li+ has been studied by means of hybrid quantum-mechanical molecular mechanical molecular dynamics simulations at Hartree-Fock and density-functional level of theory. The size of the quantum-mechanical region and the form of the potential function are shown to be of crucial importance for reliable results. Radial distribution functions, coordination number distributions, and various angular distributions have been used to discuss details of the hydration structure, together with bond lengths and bond angles of the water molecules in the first hydration shell. The lithium ion is found to be mainly fourfold coordinated with some smaller amounts of fivefold coordination. The lithium-water cluster exhibits a nearly perfect tetrahedral but still very flexible structure, in which coordinated water molecules are considerably tilted away from planarity. Water molecules in the first hydration shell are shown to be considerably polarized compared to gas-phase structures. (C) 2002 American Institute of Physics.