where we have taken φ to denote the Poisson potential and ϵ is the dielectric constant of the medium in units of the vacuum permittivity (ϵ 0 ). Notably, if ρ is taken to be the explicit charge density field (as in Eq. 2), and ϵ = 1, then φ and ψ are formally identical. More commonly, however, φ is formulated in terms of an implicit or reduced charge density field, describing only contributions from ionic species (omitting neutral solvent molecules), and ϵ is set to the dielectric constant of the solution. In this case, φ represents an effective electrostatic potential for the ionic species in solution and is thus not generally equivalent to ψ.