Abstract
The influence of higher-order terms in the Barker–Henderson approximation to the Zwanzig perturbation expansion is examined for a Lennard-Jones fluid. We find that the error introduced by ignoring these higher-order terms is negligible compared with the error due to the replacement of the reference system by a hard-sphere system in which the diameter of the spheres ensures coincidence to only first order between the free energy of the reference system and the free energy of the hard-sphere system. In calculating the contribution to the pressure due to the perturbation, one often uses the Percus–Yevick solution for the hard-sphere reference system. The error introduced by this approximation is found to be considerable.
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