  Low dimensional spin-1/2 transition metal oxides and oxyhalides continue to
be at the forefront of research investigating nonclassical phases such as
quantum spin liquids. In this study, we examine the magnetic properties of the
oxyhalide $\text{Cu}_2\text{(OH)}_3\text{Br}$ in the botallackite structure
using first-principles density functional theory, linear spin-wave theory, and
exact diagonalization calculations. This quasi-2D system consists of
$\text{Cu}^{2+}$ $\mathrm{S} = 1/2$ moments arranged on a distorted triangular
lattice. Our exact diagonalization calculations, which rely on a
first-principles-based magnetic model, generate spectral functions consistent
with inelastic neutron scattering (INS) data. By performing computational
experiments to disentangle the chemical and steric effects of the halide ions,
we find that the dominant effect of the halogen ions is steric in the
$\text{Cu}_2\text{(OH)}_3\text{X}$ series of compounds.
