Computer simulation is employed to study the structure of a thin PP film in vacuum．A initial random parent chain is obtained from the simulation of bulk amorphous polypropylene and collapsed under two-dimension boundary conditions．Films with a thickness of ～3.5 nm are obtained for parent chain with 100 repetitive units．In the interior of PP films bulk density is obtained

while density drops off sigmoidally over a distance of ～0.8 nm near the free surfaces．The backbone bonds ale randomly oriented in the interior and markedly oriented along the film plane near the free surfaces：the onset of order corresponds roughly with the drop in the mass density from its bulk value. Because of the thin film more effectively relaxing along with the plane of the thin film

the trans state and the gauche state of the CH2—CH are also enriched compared to the bulk．The difference in the energy of a chain in the amorphous bulk (3D periodic) and in the film (2D periodic) allows us to calculate the internal energy contribution to the surface energy．