Abstract
Monte Carlo and perturbation calculations for the square well system in the fluid phase, were performed for two different range parameters (R = 1.2042, 1.3675), in the inverse temperature range 0 ? X (=ε/KBT) ? 1. First order perturbation theory reproduces the machine calculated results to a fair degree of accuracy. Comparison with the triangular well fluid shows that the square well potential is rather a severe test to the theory.