  A simple model of dimerizing hard spheres with highly nontrivial fluid-solid
phase behaviour is proposed. The model is studied using the recently proposed
resummed thermodynamic perturbation theory for central force (RTPT-CF)
associating potentials. The phase diagram has the fluid branch of the
fluid-solid coexistence curve located at a temperatures lower than those of the
solid branch. This unusual behaviour is related to the strong dependence of the
system excluded volume on the temperature, which for the model at hand
decreases with increasing temperature. This effect can be also seen for a wide
family of fluid models with an effective interaction that combines short range
attraction and repulsion at a larger distance. We expect that for sufficiently
high repulsive barrier, such systems may show similar phase behaviour.
