We investigate 35 different ion models, the definitions of which (i.e., choice of the weight function and parameter C) are provided in the Supporting Information (SI1.2). Ion radii, r ion , are varied between 0.775 Å and 2.075 Å in steps of ∆r ion = 0.05 Å. For each ion modelion radius pair, we calculate all 14 PfEFIS barrier estimates. Figure displays, for example, the results for a shell ion model of size 1.225 Å. We fit each set of data to a linear function, ion position seem to be more decisive for the energy profile along the path [and, thus, also for the (reversible) work of ion insertion into an empty host structure] than commonly ex-pected. Second, the prediction error landscape exhibits a peculiar trend. Similarly reliable predictions (similar σ ∆E ) can be obtained when the ion model changes from a diffusive point over a sphere to a shell, given that the ion radius is successively decreased. Our results suggest therefore that unexpectedly distant but spatially narrow regions around an ion are particularly important for the ion insertion work. This data-driven insight becomes physically meaningful when we compare our approach to reaction field models that have been used successfully in solvation problems for many decades now. In both cases, we create a (molecular or ionic) cavity in an electrostatic field, and, subsequently, we use the electrostatic potential (effectively) on the surface of the cavity to realistically estimate the insertion work of a particle (molecule or ion) into the surrounding medium (solvent or intercalation host material). And, in both cases, the optimal cavity size is larger than the typical molecule, atom, or ion size. Note that the reaction field theory-based prediction models are currently actively expanded to heterogeneous environments such as micelles, the (solute) surroundings of which are comparable to our intercalation host materials because they are also structured.