  Imaging Air Cherenkov Telescopes (IACTs) detect very high energetic (VHE)
gamma rays. They observe the Cherenkov light emitted in electromagnetic shower
cascades that gamma rays induce in the atmosphere. A precise reconstruction of
the primary photon energy and the source flux depends heavily on accurate Monte
Carlo (MC) simulations of the shower propagation and the detector response, and
therefore also on adequate assumptions about the atmosphere at the site and
time of a measurement. Here, we present the results of an extensive validation
of the MC simulations for an analysis chain of the H.E.S.S. experiment with
special focus on the recently installed FlashCam camera on the large 28 m
telescope. One goal of this work was to create a flexible and easy-to-use
framework to facilitate the detailed validation of MC simulations also for past
and future phases of the H.E.S.S. experiment. Guided by the underlying physics,
the detector simulation and the atmospheric transmission profiles were
gradually improved until low level parameters such as cosmic ray (CR) trigger
rates matched within a few percent between simulations and observational data.
This led to instrument response functions (IRFs) with which the analysis of
current H.E.S.S. data can ultimately be carried out within percent accuracy,
substantially improving earlier simulations.
