  We present a solver for correlated impurity problems out of equilibrium based
on a combination of the so-called auxiliary master equation approach (AMEA) and
the configuration interaction expansion. Within AMEA one maps the original
impurity model onto an auxiliary open quantum system with a restricted number
of bath sites which can be addressed by numerical many-body approaches such as
Lanczos/Arnoldi exact diagonalization (ED) or matrix product states (MPS).
While the mapping becomes exponentially more accurate with increasing number of
bath sites, ED implementations are severely limited due to the fast increase of
the Hilbert space dimension for open systems, and the MPS solver typically
requires rather long runtimes. Here, we propose to adopt a configuration
interaction approach augmented by active space extension to solve numerically
the correlated auxiliary open quantum system. This allows access to a larger
number of bath sites at lower computational costs than for plain ED. We
benchmark the approach with numerical renormalization group results in
equilibrium and with MPS out of equilibrium. In particular, we evaluate the
current, the conductance as well as the Kondo peak and its splitting as a
function of increasing bias voltage below the Kondo temperature TK. We obtain a
rather accurate scaling of the conductance as a function of the bias voltage
and temperature rescaled by TK for moderate to strong interactions in a wide
range of parameters. The approach combines the fast runtime of ED with an
accuracy close to the one achieved by MPS making it an attractive solver for
nonequilibrium dynamical mean field theory.
