The Microcanonical Ensemble computer simulation method (MCE) is used to evaluate the perturbation terms A i of the Helmholtz free energy of a Square-Well (SW) fluid. The MCE method offers a very efficient and accurate procedure for the determination of perturbation terms of discretepotential systems such as the SW fluid and surpass the standard NVT Canonical Ensemble Monte Carlo method, allowing the calculation of the first six expansion terms. Results are presented for the case of a SW potential with attractive ranges 1.1 ≤ λ ≤ 1.8. Using semiempirical representation of the MCE values for A i , we also discuss the accuracy in the determination of the phase diagram of this system.

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