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Abstract
The uncorrelated-pairs approximation (UPA) is used to calculate the Helmholtz free energy of solid argon. The potential of Barker, Fisher, and Watts is used. The effects of three-body forces and quantum corrections were included only in the harmonic approximation. The internal energy, specific heat C V, bulk modulus B T, and volume coefficient of thermal expansion were calculated. The results were compared with published Monte Carlo (MC) results for the same model. Very good agreement was obtained between the UPA and MC results. The UPA results for C V were more accurate than those obtained with improved self-consistent phonon theory.