Anomalous Hall effect as the response of the orbital momentum to the gradient of electrochemical potential

It is argued that the anomalous Hall conductivity of ferromagnetic systems is fully determined by the response of the orbital momentum given by the so-called free electron currents to the gradient of the electrochemical potential. The effect of the side-jump and skew scattering is discussed as well as the relation to the general form of the Kubo Hall-conductivity formula.