  By combining Wilson's numerical renormalization group with a modified
Bloch-Redfield approach we are able to eliminate the artificial broadening of
the Lehmann representation of quantum impurity spectral functions required by
the standard numerical renormalization group algorithm. Our approach is based
on the exact reproduction of the continuous coupling function in the original
quantum impurity model. It augments each chain site of the Wilson chain by a
coupling to an additional reservoir. This open Wilson chain is constructed by a
continuous fraction expansion and the coupling function is treated in second
order in the context of the Bloch-Redfield approach. The eigenvalues of the
resulting Bloch-Redfield tensor generate a finite life time of the numerical
renormalization group excitations that leads to a natural broadening of the
spectral functions. We combine this approach with z-averaging and an analytical
exact expression for the correlation part of the self-energy to obtain an
accurate representation of the spectral function of the original continuum
model in the absence and presence of an external magnetic field.
