(Received: June 22, 2005; Accepted for publication: November 30, 2005; Published on Web: March 3, 2006)
The canonical ensemble partition function is obtained for a two-molecule system with the repulsive step function outside the hard sphere wall in a spherical cell. Fluid-fluid phase transition-like behavior is evident at low temperatures. The spinodal line is obtained in the (V, T) domain and a negative expansion coefficient is found in the stable state. The theoretical pVT relationship is confirmed through Monte Carlo simulations. The periodic 108-molecule system examined by Monte Carlo simulations also provides a qualitatively similar pVT relationship to the two-molecule system.
Keywords: Negative Expansion, Canonical ensemble partition function, Repulsive step function, Monte Carlo simulations, Spinodal line