This work uses a full Monte Carlo simulation of the HIST, including modeled geometry and electronics to derive the instrument response to electrons up to 12 MeV and protons to 120 MeV. These simulations use a flat, continuous particle energy spectrum as a normally incident beam and are validated by comparisons with pre-launch beam calibrations. On-orbit characteristics and efficiencies are determined using a realistic power law energy spectrum. Since exhaustive calibrations can be prohibitively expensive, such simulations augment and complement calibrations. This validated simulation tool is used to compute the response of HIST in all operating modes and in turn optimize the instrument. The flight data are corrected by the computed response to give clean spectra which the provide the basis for all subsequent analysis.