  The existence of a "non-Markovian dissipationless" regime, characterized by
long lived oscillations, was recently reported for a class of quantum open
systems (Zhang et al, PRL, 109, 170402, (2012)). It is claimed this could
happen in the strong coupling regime, a surprising result which has attracted
some attention. We show that this regime exists if and only if the total
Hamiltonian is unbounded from below, casting serious doubts on the usefulness
of this result.
