Structure and dynamics of the Li⁺ hydrates: A comparative study of conventional QM/MM and ONIOM-XS MD simulations

The hydration structure and dynamics of Li⁺ in liquid water have been investigated by means of two combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) techniques, namely a conventional QM/MM MD and a more sophisticated QM/MM MD based on the ONIOM-XS method, called briefly ONIOM-XS MD. Based on the two QM/MM-based MD simulations, the feature of the first hydration shell of Li⁺ is almost identical, showing a well-defined tetrahedral geometry with the average coordination numbers of 4.1 and 4.2, respectively. However, significant differences between the conventional QM/MM and ONIOM-XS MD simulations appear in the detailed analyses of the geometrical arrangement and the dynamics of the Li⁺ hydrates. As compared to the conventional QM/MM MD study, the ONIOM-XS MD simulation clearly reveals that the structure of the hydrated Li⁺ is more flexible and that water molecules in the first hydration shell are more labile, leading to a higher probability of finding other hydrated complexes, in particular the Li⁺(H₂O)₅ species. In this respect, the ONIOM-XS MD results clearly imply that the "structure-making" ability of this ion in aqueous solution is not too strong.