Magnetic lattice gas of molecules with internal structure

A model of a lattice gas is considered in which molecules interact via three kinds of forces: short-ranged intra-molecular magnetic (producing a net magnetic moment per particle), long-ranged magnetic between those moments and finally long-ranged non-magnetic. Using the molecular field approximation for both kinds of the long-ranged forces and calculating exactly the contribution from the short-ranged interactions, we construct and discuss phase diagrams for the system. We show that the location of the (multi-) critical points is quite sensitive to the strength and/or sign of the intra-molecular interaction, as well as to the form of the molecules.