The structures and thermodynamic properties of the (100), (010), and (110) lateral surfaces of extended-chain polyethylene crystals between 0 and 300 K were determined by minimization of vibrational free energy using consistent quasi-harmonic lattice dynamics. Slight rotations of the outermost chains from their corresponding orientations in the bulk were observed. These deviations from bulk structure were confined to the first three molecular layers (approximately 10 Angstrom) at the surface. Surface free energy calculations for the crystal-vacuum interface found the (110) surface to be the most stable over the entire temperature range modeled, with free energies ranging from 95.4 erg/cm(2) to 103.5 erg/cm(2) at temperatures of 0 and 300 K, respectively. Surface free energies of the (100) and (010) surfaces were found to be at least 15% higher than the (110) surface, with the (100) surface slightly more stable than the (010) surface, over all temperatures considered. Surface free energy increases as the density of chains at the surface decreases. Intermolecular potential energy accounted for over 80% of the surface free energy at low temperatures. At higher temperatures, excess entropy accounted for almost half the surface free energy. The surface free energy was determined predominantly by deviations in lattice mode frequencies from those of the bulk.