For fair comparison, this function must correspond reasonably well with a model in which a dielectric constant (either ε solv or ε nonp ) is assigned to each atomic sphere at the user's discretion. To construct a corresponding function ε(r), we use Becke's definition of "fuzzy" Voronoi cells, 102 as described in Appendix A, to define ε(r) in pointwise fashion over the real-space grid that is used in PEqS calculations. The weight function w A (r) that is defined in eq. A14 is used to smooth the boundaries of the Voronoi cells defined by atom (nucleus) A, and then we impose that ε(r) = 1 for all points inside of the molecular cavity. The result is a "primitive" permittivity function