UNCORRELATED-FACTORS APPROXIMATION AND A COMPARISON OF THEORIES FOR PREDICTING THERMAL-PROPERTIES - A LENNARD-JONES SOLID

The equations for determining the free energy of a solid with two-body interactions in the uncorrelated-factors approximation (UFA) are deri ved from the correlated-factors theorem. A self-consistent choice of t he parameters in the harmonic Hamiltonian causes the approximation to be accurate through second order. The specific heat, thermal expansion , and bulk modulus of an fcc Lennard-Jones solid with nearest-neighbor interactions only are calculated in the UFA and the results are compa red with the predictions of lowest-order and improved self-consistent phonon theory (SC1 and ISC), perturbation theory through fourth order, and other approximations. The predictions of the UFA are in very good agreement with new classical Monte Carlo estimates and with recent ef fective potential Monte Carlo results. The calculational effort requir ed in the UFA is similar to that in SC1, while the accuracy of the pre dictions is similar to that of ISC.