In the non-relativistic limit, ∂ ∂ t χ χ and the kinetic and electrostatic energies are small with respect to the rest energy m c 2 , leading to the Lévy-Leblond equation. Thus χ χ ≈ ≈ σ σ ⋅ ⋅ Π Π ψ ψ 2 m c .