  Local three- and four-point correlators yield important insight into strongly
correlated systems and have many applications. However, the nonperturbative,
accurate computation of multipoint correlators is challenging, particularly in
the real-frequency domain for systems at low temperatures. In the accompanying
paper, we introduce generalized spectral representations for multipoint
correlators. Here, we develop a numerical renormalization group (NRG) approach,
capable of efficiently evaluating these spectral representations, to compute
local three- and four-point correlators of quantum impurity models. The key
objects in our scheme are partial spectral functions, encoding the system's
dynamical information. Their computation via NRG allows us to simultaneously
resolve various multiparticle excitations down to the lowest energies. By
subsequently convolving the partial spectral functions with appropriate
kernels, we obtain multipoint correlators in the imaginary-frequency Matsubara,
the real-frequency zero-temperature, and the real-frequency Keldysh formalisms.
We present exemplary results for the connected four-point correlators of the
Anderson impurity model, and for resonant inelastic x-ray scattering (RIXS)
spectra of related impurity models. Our method can treat temperatures and
frequencies -- imaginary or real -- of all magnitudes, from large to
arbitrarily small ones.
