The spin–orbit interaction refers to the interaction between the intrinsic magnetic moment of the electron with the magnetic field experienced by it due to the relative motion with the proton. The interaction Hamiltonian is H s o = − − e m c m ⋅ ⋅ L r 3 = e 2 m 2 c 2 r 3 S ⋅ ⋅ L which may be written as H s o = e 2 4 m 2 c 2 r 3 [ J 2 − − L 2 − − S 2 ]