MOLECULAR-DYNAMICS AND HIGHER-ORDER PERTURBATION-THEORY RESULTS FOR THE ANHARMONIC FREE-ENERGY AND EQUATION OF STATE OF A LENNARD-JONES SOLID

The anharmonic contribution to the Helmboltz free energy (F-A) is calc ulated as a function of temperature (T) and volume (V) fora fee crysta l of atoms interacting via a nearest-neighbor 6-12 Lennard-Jones poten tial, by the molecular-dynamics (MD) method and by perturbation theory (PT) to O(lambda(2)) and O(lambda(4)). The volume-dependent coefficie nts for the T-2 and T-3 terms of F-A have been extracted from the MD r esults, and compared with the corresponding coefficients of lambda(2) and lambda(4) PT. The agreement between the results of the MD and PT m ethods is excellent. The coefficient of the T-4 term of FA is also ext racted from the MD results, to give an approximation to the correspond ing term in O(lambda(6)) pT. Various thermodynamic properties are calc ulated from this approximation of PT top(lambda(6)), and are compared to exact MD results to provide a picture of the convergence properties of PT. The results suggest that lambda(6) PT would be accurate up to approximately 60% of the melting temperature.