The problem is a simple n-body problem: there is n charged particles - electrons, protons, or other particles; we have the initial parameters: a mass, charge, position, velocity, at a time t = 0. In a standard (classical) numerical computation of the n-body problem I can use any numerical method based on a force, say: F = am = k/r^2; for gravity and for the charged bodies is the same... But I want to apply the SR model.. so, what is a difference - the force formula has other form - how is it? The SR equation is still: E = Ek + Ep the Ek has other form: [tex]E_k = mc^2(\gamma-1)[/tex] and the potential Ep is the same: Ep = k/r; but what is the force which I want to apply in every step of the simulation? F_ij = m_i ai = ... ?