In this work, the method of isoconfigurational ensemble is applied to an entangled polymer melt. Based on the interpretation of the path of a molecule through a melt as the mean position of its monomers, this method allows us to noninvasively find the confining tube of a molecule in terms of size, shape, primitive path, and statistical properties. This analysis was also used to explore the effects of a free surface on the tube. It was found that the tube diameter smoothly increases by about 25%, and the surface effect is felt to about two tube diameters deep. The surface also induces anisotropy in the tube; segments within about a tube diameter of the surface tend to lie parallel to the surface.
