Sum-rule Conserving Spectral Functions from the Numerical Renormalization Group
Andreas Weichselbaum
Ludwig-Maximilians-Universität
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We show how spectral functions for quantum impurity models can
be calculated very accurately using a complete set of ``discarded''
numerical renormalization group eigenstates, recently introduced by
Anders and Schiller. The only approximation is to judiciously exploit
energy scale separation. Our derivation avoids both the overcounting
ambiguities and the single-shell approximation for the equilibrium
density matrix prevalent in current methods, ensuring that relevant sum
rules hold rigorously and spectral features at energies below the
temperature can be described accurately