  The hierarchical quantum master equation (HQME) approach is an accurate
method to describe quantum transport in interacting nanosystems. It generalizes
perturbative master equation approaches by including higher-order contributions
as well as non-Markovian memory and allows for the systematic convergence to
the numerically exact result. As the HQME method relies on a decomposition of
the bath correlation function in terms of exponentials, however, its
application to systems at low temperatures coupled to baths with complexer band
structures has been a challenge. In this publication, we outline an extension
of the HQME approach, which uses a re-summation over poles and can be applied
to calculate transient currents at a numerical cost that is independent of
temperature and band structure of the baths. We demonstrate the performance of
the extended HQME approach for noninteracting tight-binding model systems of
increasing complexity as well as for the spinless Anderson-Holstein model.
