In this work we examine thermodynamics of fluid with "molecules" represented
by two fused hard spheres, decorated by the attractive square-well sites.
Interactions between these sites are of short-range and cause association
between the fused-sphere particles. The model can be used to study the
non-spherical (or dimerized) proteins in solution. Thermodynamic quantities of
the system are calculated using a modification of Wertheim's thermodynamic
perturbation theory and the results compared with new Monte Carlo simulations
under isobaric-isothermal conditions. In particular, we are interested in the
liquid-liquid phase separation in such systems. The model fluid serves to
evaluate the effect of the shape of the molecules, changing from spherical to
more elongated (two fused spheres) ones. The results indicate that the effect
of the non-spherical shape is to reduce the critical density and temperature.
This finding is consistent with experimental observations for the antibodies of
non-spherical shape.Comment: 12 pages, 5 figure