where δ is the Dirac delta function. As P MRF 2,λ ([ρ]; r, u) is given by the sum of δ functions, it is not meant to approximate the exact P 2,λ ([ρ]; r, u) at finite λ values. Instead, it only serves as the auxiliary object that delivers w MRF λ (r), since this is the object that MRF aims to accurately approximate. In other words, P MRF 2,λ ([ρ]; r, u) is not an approximation of the exact P 2,λ ([ρ]; r, u) at finite λ, but the electrostatic potential of the former is an approximation to that of the latter.