  In order to realize the significant potential of optical materials such as
metal halides, computational techniques which give accurate optical properties
are needed, which can work hand-in-hand with experiments to generate high
efficiency devices. In this work a computationally efficient technique based on
semiconductor Bloch equations (SBEs) is developed and applied to the material
BiSBr. This approach gives excellent agreement with the experimental optical
gap, and also agrees closely with the excitonic stabilisation energy and the
absorption spectrum computed using the far more computationally demanding
\textit{ab initio} Bethe-Salpeter approach. The SBE method is a good candidate
for theoretical spectroscopy on large- or low dimensional systems which are too
computationally expensive for an \textit{ab initio} treatment.
