  HERMES (High Energy Rapid Modular Ensemble of Satellites) is an innovative
mission aiming to observe transient high-energy events such as gamma-ray bursts
(GRBs) through a constellation of CubeSats hosting a broadband X and gamma-ray
detector. The detector is based on a solid-state Silicon Drift Detector (SDD)
coupled to a scintillator crystal, and is sensitive in the 2 keV to 2 MeV band.
An accurate evaluation of the foreseen in-orbit instrumental background is
essential to assess the scientific performance of the experiment. An outline of
the Monte Carlo simulations of the HERMES payload will be provided, describing
the various contributions on the total background and the optimization
strategies followed in the instrument design. Moreover, the simulations were
used in order to derive the effective area and response matrices of the
instrument, also as a function of the source location with respect to the
detector frame of reference.
