  The effect of anisotropy in the energy spectrum on the binding energy and
structural properties of excitons, trions, and biexcitons is investigated. To
this end we employ the stochastic variational method with a correlated Gaussian
basis. We present results for the binding energy of different excitonic
complexes in black phosphorus (bP) and TiS$_3$ and compare them with recent
results in the literature when available, for which we find good agreement. The
binding energies of excitonic complexes in bP are larger than those in TiS$_3$.
We calculate the different average interparticle distances in bP and TiS$_3$
and show that excitonic complexes in bP are strongly anisotropic whereas in
TiS$_3$ they are almost isotropic, even though the constituent particles have
an anisotropic energy spectrum. This is also confirmed by the correlation
functions.
