On the role of density fluctuations in the equation of state of a simple fluid

We address the wellknown problems introduced into the theory of fluids by d ensity fluctuations in the form of van der Waals loops and nonclassical cri tical phenomena. A clean separation of long and short range density fluctua tions is achieved by use of cell-constrained models which display well-defi ned van der Waals loops and classical behaviour around the critical point. For a pure Lennard-Jones fluid with occupancy restricted to 1 or 8 particle s per cell, the phase diagram is determined by Monte Carlo simulation. By c onsidering the deviations from the normal simulations without cell constrai nt, the effects of longer range density fluctuations are exposed. The syste m size dependence of the van der Waals loops present in all simulations of fluids is analyzed in terms of the GvdW free energy density functional theo ry, which is formulated on the basis of the cell concept. The loops are fou nd to gradually disappear either with greater cell occupancy or increasing total particle number in the simulation box.