  Polymeric thin films of various thicknesses, confined between two repulsive
walls, have been studied by molecular dynamics simulations. Using the
anisotropy of the perpendicular and parallel components of the pressure tensor
the surface tension of the system is calculated for a wide range of temperature
and for various film thicknesses. Three methods of determining the pressure
tensor are compared: the method of Irving and Kirkwood (IK), an approximation
thereof (IK1), and the method of Harasima (H). The IK- and the H-methods differ
in the expression for the parallel component, but not for the normal component
of the pressure tensor. Both methods yield a constant normal component
throughout the film, as required by mechanical stability, whereas the
IK1-method leads to strong oscillations. However, all methods give the same
expression for the surface tension.
