The thermodynamic perturbation dimer theory (TPT-D) is modified by scaling the site-site correlation function at contact for hard-dimers using the contact value of site-site correlation function for hard-spheres. With the exception of a constant, the resulting modified TPT-D is identical with the thermodynamic perturbation theory (TPT). The modified TPT-D, however, predicts the Monte Carlo (MC) compressibility factors for hard-sphere chains more accurately than TPT. The average absolute percent deviations (%AAD) was found to be 2.75 for the modified TPT-D and 6.24 for TPT. The modified TPT-D was then extended to mixtures of hard-sphere chain fluids. The modified TPT-D accurately predicts compressibility factors for binary mixtures of hard-sphere chains or mixtures containing hard-sphere chain and hard-sphere fluids. When the theories were used to predict compressibility factors for 89 mixtures containing hard-spheres and hard-sphere chains with different lengths and number of segments the %AAD was found to be 1.49 for the modified TPT-D and 2.78 for TPT.
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