where δρ(z)| D = ρ D (z) -ρ D=0 (z) is the response charge density. It is found that the image plane position z im (relative to the atomic plane z a ) is 1.22 Å for Au(100) and 1.49 Å for Before discussing the impact of the image plane on the total capacitance, one may wonder how to validate the physical significance of z im computed using Eq. 2. Lang and Kohn showed that the definition of image plane according to Eq. 2 recovers the classical expression for the image potential. Here we looked into this question by thinking how dielectric response works for metal. A metal should completely screen the electric field. This means the following relation must hold true: