Average solvation structure of K⁺ in liquid water (the radial distribution function g(r) of the ion–oxygen pair) at 298 K. Results are shown for the non-polarizable TIP3P/CHARMM force field and Drude polarizable force field (SWM4-NDP water model; Lamoureux et al., 2006) as well as two ab initio CPMD models. CPMD (version 3.13.3) simulations were performed using a cubic periodic cell with a length of 12.41 Å containing 64 water molecules and one K⁺ ion. The electronic structure was calculated with the BLYP functional (Becke, 1988; Lee et al., 1988), Troullier and Martins pseudopotentials (Troullier and Martins, 1991), and a plane-wave cutoff of 80 Ry. Also included are CPMD results with the BLYP function with an empirical correction (+D) for van der Waals dispersion (Grimme, 2006). CPMD results were generated by a 20-ps equilibration followed by a 30-ps trajectory for analysis, using a time step of 0.12 fs. Each CPMD simulation was initiated from an equilibrated Drude force field simulation, as described by Whitfield et al. (2007). The multiple gray lines for the neutron scattering correspond to experiments on solutions of different potassium-halide salts (Soper and Weckström, 2006).