To demonstrate our proposal, we considered two Na+ atoms close to each other. We employed a simulation model consisting of 16∆x × 16∆x × 16∆x grids (∆x = 0.3 Å). We set two Na+ atoms separated 1 Å at the centre of this model, which were converted into densities with σ = 0.3 Å. Then, we solved the equation ( ) and ( ) by Fourier spectral method with time step ∆t = 10 fs. We employed q A = q B = 1e, κ A = κ B = 5.19 × 10 -8 m 2 /(V • s). As ϵ 0 is the dielectric constant of vacuum 8.85 × 10 -12 F/m, ϵ = 78.5ϵ 0 was taken assuming surrounding water.