Recent atomistic simulations of polyethylene melt afford a close look at the entanglement dynamics of a real polymer. We analyze these results in a new way, by defining the tube primitive path as the mean path of consecutive molecular dynamics trajectories. The result suggests that tube is semiflexible, the persistence length being about half the entanglement length. The time dependent tangent-tangent correlation function of tube primitive path is then used to test the standard molecular model of tube dynamics. It is found that the effect of the semiflexibility is important in the mildly entangled system we studied and that incorporating its effect into the standard tube model improves the theory drastically. (C) 2012 The Society of Rheology. [http://dx.doi.org/10.1122/1.4708594]