Delocalization error constrains the accuracy of density functional theory (DFT) in describing molecular interactions in ion–water systems. Using Na+ and Cl− in water as model systems, we calculate the effects of delocalization error in the SCAN functional for describing ion–water and water–water interactions in hydrated ions, and demonstrate that density-corrected SCAN (DC-SCAN) predicts n-body and interaction energies with an accuracy approaching coupled cluster theory. The performance of DC-SCAN is size-consistent, maintaining an accurate description of molecular in- teractions well beyond the first solvation shell. Molecular dynamics simulations at ambient conditions with MB-SCAN(DC) potentials, derived from the many-body expansion, predict the solvation structure of Na+ and Cl− in quantitative agreement with reference data, while simultaneously reproducing the structure of liquid water. Beyond rationalizing the accuracy of density-corrected models of ion hydration, our findings suggest that our unified density-corrected many-body formalism holds great promise for efficient DFT-based simulations of condensed-phase systems with chemical accuracy.
Methods. Additional analyses and figures.