We present a new method to calculate directly the one-particle self-energy of
an impurity Anderson model with Wilson's numerical Renormalization Group method
by writing this quantity as the ratio of two correlation functions. This way of
calculating Sigma(z) turns out to be considerably more reliable and accurate
than via the impurity Green's function alone. We show results for the
self-energy for the case of a constant coupling between impurity and conduction
band (ImDelta = const) and the effective Delta(z) arising in the Dynamical Mean
Field Theory of the Hubbard model. Implications to the problem of the
metal-insulator transition in the Hubbard model are also discussed.Comment: 18 pages, 9 figures, submitted to J. Phys.: Condens. Matte