  We use the Matsubara functional renormalization group (FRG) to describe
electronic correlations within the single impurity Anderson model. In contrast
to standard FRG calculations, we account for the frequency-dependence of the
two-particle vertex in order to address finite-energy properties (e.g, spectral
functions). By comparing with data obtained from the numerical renormalization
group (NRG) framework, the FRG approximation is shown to work well for
arbitrary parameters (particularly finite temperatures) provided that the
electron-electron interaction U is not too large. We demonstrate that aspects
of (large U) Kondo physics which are described well by a simpler
frequency-independent truncation scheme are no longer captured by the
'higher-order' frequency-dependent approximation. In contrast, at small to
intermediate U the results obtained by the more elaborate scheme agree better
with NRG data. We suggest to parametrize the two-particle vertex not by three
independent energy variables but by introducing three functions each of a
single frequency. This considerably reduces the numerical effort to integrate
the FRG flow equations.
