  Recent developments in the numerical renormalization group (NRG) allow the
construction of the full density matrix (FDM) of quantum impurity models (see
A. Weichselbaum and J. von Delft) by using the completeness of the eliminated
states introduced by F. B. Anders and A. Schiller (2005). While these
developments prove particularly useful in the calculation of transient response
and finite temperature Green's functions of quantum impurity models, they may
also be used to calculate thermodynamic properties. In this paper, we assess
the FDM approach to thermodynamic properties by applying it to the Anderson
impurity model. We compare the results for the susceptibility and specific heat
to both the conventional approach within NRG and to exact Bethe ansatz results.
We also point out a subtlety in the calculation of the susceptibility (in a
uniform field) within the FDM approach. Finally, we show numerically that for
the Anderson model, the susceptibilities in response to a local and a uniform
magnetic field coincide in the wide-band limit, in accordance with the
Clogston-Anderson compensation theorem.
