where a new parameter z0 appears denoting the position of the image plane. Moreover, we may define the center of mass of the charge-density correction due to the external electric field zc, and in this particular case, those two parameters occur to be equal z0 = zc. However, in 1995 Finnis et al. found that those two parameters are indeed distinct, and they only coincide in the limit of the linear response theory -that is, the charge center of mass zc depends via the non-linear response on q, whereas the image plane z0 stays independent. Moreover, the image potential with the non-linear quartic term takes the form