The surface free energy, gamma(mu), calculated from the chemical potential, mu, and the contribution of the internal energy to the surface energy, gamma(U), are compared for a molecular model of a free-standing thin film of n-tetracontane. The coarse-grained chains are represented on a high-coordination lattice with a step length of 2.5 Angstrom. Each occupied site on this lattice represents a two-carbon unit of n-tetracontane. The configurations of the coarse-grained chains are governed by two types of energies. Short-range intramolecular conformations are weighted using first- and second-order interactions in the classic rotational isomeric state model of Abe et al. for polyethylene (J. Am. Chem. Soc. 1966, 88, 63 1). Intermolecular, as well as long-range intramolecular, interactions are weighted with a discretized Lennard-Jones potential energy function, truncated after the third shell. The contribution of these two energies to gamma(U) of the amorphous free-standing thin,film is 22 erg/cm(2) at 400 K. The chemical potential of the chains in the thin film is evaluated using the chain increment method proposed by Kumar et al. (Phys. Rev. Lett. 1991, 66, 2935). The analysis of the chemical potential specifies gamma(mu) 20 erg/cm(2) at 400 K. The estimates of gamma(U) and gamma(mu) at this temperature, T, differ by about 10%. The small difference in these two values implies that the energetic contributions dominate the entropic contributions to gamma(mu) of this amorphous free-standing thin film. Both gamma(U) and gamma(mu) decrease, at nearly the same rate, when T of the simulation rises above 400 K. With a reduction in T below 390 K, crystallization of the thin film occurs. The value of gamma(U) increases as T falls below 390 K, reaching 32 erg/cm(2) at 298 K. The usual implementation of the chain increment method for calculating mu becomes unreliable when crystals appear in the thin film.