  The computation of transport coefficients, even in linear response, is a
major challenge for theoretical methods that rely on analytic continuation of
correlations functions obtained numerically in Matsubara space. While maximum
entropy methods can be used for certain correlation functions, this is not
possible in general, important examples being the Seebeck, Hall, Nernst and
Reggi-Leduc coefficients. Indeed, positivity of the spectral weight on the
positive real-frequency axis is not guaranteed in these cases. The spectral
weight can even be complex in the presence of broken time-reversal symmetry.
Various workarounds, such as the neglect of vertex corrections or the study of
the infinite frequency or Kelvin limits have been proposed. Here, we show that
one can define auxiliary response functions that allow to extract the desired
real-frequency susceptibilities from maximum entropy methods in the most
general multiorbital cases with no particular symmetry. As a benchmark case, we
study the longitudinal thermoelectric response and corresponding Onsager
coefficient in the single-band two-dimensional Hubbard model treated with
dynamical mean-field theory (DMFT) and continuous-time quantum Monte Carlo
(CTQMC). We thereby extend to transport coefficients the maximum entropy
analytic continuation with auxiliary functions (MaxEntAux method), developed
for the study of the superconducting pairing dynamics of correlated materials.
