In order to calculate the electrostatic contribution φ, discreteness of sites must be taken into account . Following, Ref [98], we can split φ into the mean-field and discrete surface charge contributions βqφ = βqϕ(a) + βqϕ disc . The mean-field ϕ(a) is determined by the solution of the PB equation, while the effects due to the discreteness of the surface sites are included using the theory of one component plasma (OCP) resulting in 98