Fluid–fluid Transition and Negative Expansion in 2 Step-function Molecules System by Statistical Mechanics

In order to perform the statistical mechanical calculations, we adopt the periodic cubic system with two molecules in the unit cell. Our model potential function consists of a step-function and the hard sphere wall. We assume the minimum image convention and get the canonical partition function. We find the fluid–fluid phase transition and the negative thermal expansion in the system. We discuss the thermodynamic properties vs. temperature plots under the constant volume. We show some results of the Monte Carlo (MC) simulations on the system with the same potential function under the periodic cubic boundary condition for a comparison. The theoretical results on pVT relations are in agreement with the MC simulations on the two-molecule system. The results of 108-molecule system with the MC simulations are expected to have the similar phase transition near the state in the case of the two-molecule system.