Using the Feynman rules to compute the S-matrix element, we obtain in the non-relativistic limit with m 0 ≫ ≫ | p | ⟨ ⟨ p ′ , k | S | p , k ⟩ ⟩ | c o n n = − − i e 2 | p − − p ′ | 2 − − i ε ε ( 2 m ) 2 δ δ ( E p , k − − E p ′ , k ) ( 2 π π ) 4 δ δ ( p − − p ′ )