  We investigate thermodynamic properties of anisotropic colloidal dumbbells in
the frameworks provided by the Reference Interaction Site Model (RISM) theory
and an Optimized Perturbation Theory (OPT), this latter based on a fourth-order
high-temperature perturbative expansion of the free energy, recently
generalized to molecular fluids. Our model is constituted by two identical
tangent hard spheres surrounded by square-well attractions with same widths and
progressively different depths. Gas-liquid coexistence curves are obtained by
predicting pressures, free energies, and chemical potentials. In comparison
with previous simulation results, RISM and OPT agree in reproducing the
progressive reduction of the gas-liquid phase separation as the anisotropy of
the interaction potential becomes more pronounced; in particular, the RISM
theory provides reasonable predictions for all coexistence curves, bar the
strong anisotropy regime, whereas OPT performs generally less well. Both
theories predict a linear dependence of the critical temperature on the
interaction strength, reproducing in this way the mean-field behavior observed
in simulations; the critical density~--~that drastically drops as the
anisotropy increases~--~turns to be less accurate. Our results appear as a
robust benchmark for further theoretical studies, in support to the simulation
approach, of self-assembly in model colloidal systems.
