  The non-Markovian dynamics of open quantum systems is still a challenging
task, particularly in the non-perturbative regime at low temperatures. While
the Stochastic Liouville-von Neumann equation (SLN) provides a formally exact
tool to tackle this problem for both discrete and continuous degrees of
freedom, its performance deteriorates for long times due to an inherently
non-unitary propagator. Here we present a scheme which combines the SLN with
projector operator techniques based on finite dephasing times, gaining
substantial improvements in terms of memory storage and statistics. The
approach allows for systematic convergence and is applicable in regions of
parameter space where perturbative methods fail, up to the long time domain.
Findings are applied to the coherent and incoherent quantum dynamics of two-
and three-level systems. In the long time domain sequential and super-exchange
transfer rates are extracted and compared to perturbative predictions.
