  We study the Kern-Frenkel model for patchy colloids using Barker-Henderson
second-order thermodynamic perturbation theory. The model describes a fluid
where hard sphere particles are decorated with one patch, so that they interact
via a square-well (SW) potential if they are sufficiently close one another,
and if patches on each particle are properly aligned. Both the gas-liquid and
fluid-solid phase coexistences are computed and contrasted against
corresponding Monte-Carlo simulations results. We find that the perturbation
theory describes rather accurately numerical simulations all the way from a
fully covered square-well potential down to the Janus limit (half coverage). In
the region where numerical data are not available (from Janus to hard-spheres),
the method provides estimates of the location of the critical lines that could
serve as a guideline for further efficient numerical work at these low
coverages. A comparison with other techniques, such as integral equation
theory, highlights the important aspect of this methodology in the present
context.
