  The single impurity Anderson model (SIAM) is studied within an enhanced
non-crossing approximation (ENCA). This method is extended to the calculation
of susceptibilities and thoroughly tested, also in order to prepare
applications as a building block for the calculation of susceptibilities and
phase transitions in correlated lattice systems. A wide range of model
parameters, such as impurity occupancy, temperature, local Coulomb repulsion
and hybridization strength, are studied. Results for the spin and charge
susceptibilities are presented. By comparing the static quantities to exact
Bethe ansatz results, it is shown that the description of the magnetic
excitations of the impurity within the ENCA is excellent, even in situations
with large valence fluctuations or vanishing Coulomb repulsion. The description
of the charge susceptibility is quite accurate in situations where the singly
occupied ionic configuration is the unperturbed ground state; however, it seems
to overestimate charge fluctuations in the asymmetric model at too low
temperatures. The dynamic spin excitation spectra is dominated by the
Kondo-screening of the impurity spin through the conduction band, i.e. the
formation of the local Kondo-singlet. A finite local Coulomb interaction U
leads to a drastic reduction of the charge response via processes involving the
doubly occupied impurity state. In the asymmetric model, the charge
susceptibility is enhanced for excitation energies smaller than the Kondo scale
T_K due to the influence of valence fluctuations.
