Ion-specific hydration structures revealed by SCAN-based <i>ab initio</i> simulations

Abstract Hydrated ions play essential roles in diverse chemical and biological processes, yet accurately characterizing their hydration structures remains challenging due to the delicate interplay of ion–water and water–water interactions. Here, we use ab initio molecular dynamics (AIMD) simulations based on the strongly constrained and appropriately normed (SCAN) exchange–correlation functional to systematically investigate the hydration structures of eight representative ions (Mg 2+ , Ca 2+ , Li + , Na + , K + , F − , Cl − , Br − ) in aqueous solution. Compared to the widely used Perdew–Burke–Ernzerhof (PBE) functional, SCAN substantially improves the description of solvent water by weakening the hydrogen-bond network and enhancing structural disorder, yielding results in closer agreement with experiments. SCAN modifies ionic hydration shells in an ion-specific manner, governed by ionic size and charge, and reproduces experimental hydration geometries especially well for intermediate-size monovalent ions (Na + , Cl − ). Moreover, SCAN consistently reduces the overpolarization of water molecules near ions. These improvements lead to more accurate and physically consistent hydration structures, highlighting SCAN’s utility for modeling complex aqueous systems and offering guidance for future studies of ionic solvation.