  We present a hierarchical equations of motion approach, which allows a
numerically exact simulation of nonequilibrium transport in general open
quantum systems involving multiple macroscopic bosonic and fermionic
environments. The performance of the method is demonstrated for a model of a
nanosystem, which involves interacting electronic and vibrational degrees of
freedom and is coupled to fermionic and bosonic baths. The results show the
intricate interplay of electronic and vibrational degrees of freedom in this
nonequilibrium transport scenario for both voltage and thermally driven
transport processes. Furthermore, the use of importance criteria to improve the
efficiency of the method is discussed.
