Coordination number and equation of state of square-well and square-shoulder fluids: simulation and quasi-chemical model

Abstract

Extensive Monte Carlo (MC) computer simulations of square-well fluids with variable well-width have been performed to determine the equation of state and local coordination numbers. An accurate yet simple analytically derived equation of state for square-well fluids is obtained using the quasi-chemical approximation (QCA) and empirically refined using the MC data. This extends the range of earlier models using this approach to arbitrary well width and sign of the well (either attractive well or repulsive shoulder). We also demonstrate, for the first time, that the present QCA formula also reproduces well the coordination number and the equation of state of the square-shoulder (SS) fluid over a wide density, temperature and well width range. The absence of a critical point for the SS fluid enables the simple QCA expressions to be more successful over a wider temperature range than those of the attractive square wells.