Helmholtz free energy of an anharmonic crystal toO(λ4). II

Numerical calculations of the Helmholtz free energy $F$ to $O\left({\lambda }^2\right)$ and $O\left({\lambda }^4\right)$ from all the diagrams have been carried out in the high-temperature limit without making the leading-term approximation for a monatomic face-centered cubic crystal with nearest-neighbor central-force interactions. The numbers obtained for some diagrams and the total $F\left({\lambda }^4\right)$ can differ by as much as 47 and 33%, respectively from those obtained in the leading term approximation, indicating that this approximation is not very good as far as absolute magnitudes are concerned. However, the ratio $\frac{F\left({\lambda }^4\right)}{F\left({\lambda }^2\right)}$ is nearly the same as in the leading-term approximation, indicating that the convergence of the perturbation expansion is satisfactory up to one third of the melting temperature. Finally, the improved-self-consistent (ISC) scheme of selecting the most important diagrams is probably as good as doing perturbation theory to order ${\lambda }^4$.