A simulator for quantum information systems cannot be both general,
that is, easily used for every possible system, and efficient.
Therefore, some systems will have aspects which can only be simulated
by cunning modeling. On the other hand, a simulation may conveniently
do extra-systemic processing that would be impractical in a real system.
We illustrate with examples from our quantum computing simulator, QCSim.
We model the [3,1] Hamming code in the
presence of random bit flip or generalized amplitude damping noise,
and calculate the expected result in one simulation run, as opposed
to, say, a Monte Carlo simulation, and keep the original state to
compute the chance of successful transmission, too. We also model the
BB84 protocol with eavesdropping and random choice of basis and
compute the chance of information received faithfully. Finally, we
present our simulation of teleportation as an example of the trade-off
between complexity of the simulation model and complexity of
simulation inputs and as an example of modeling
measurements and classical bits.