  We present results of Monte Carlo simulations measuring the coherent
structure function of a chain moving through an ordered lattice of fixed
topological obstacles. Our computer experiments use chains up to 320 beads and
cover a large range of wave vectors and a time range exceeding the reptation
time. -- We compare our results (i) to the predictions of the primitive chain
model, (ii) to an approximate form resulting from Rouse motion in a coiled
tube, and (iii) to our recent evaluation of the full reptation model. (i) The
primitive chain model can fit the data for times t \gt 20 T_2, where T_2 is the
Rouse time of the chain. Besides some phenomenological amplitude factor this
fit involves the reptation time T_3 as a second fit parameter. For the chain
lengths measured, the asymptotic behavior T_3 ~ N^3 is not attained. (ii) The
model of Rouse motion in a tube, which we have criticized before on theoretical
grounds, is shown to fail also on the purely phenomenological level. (iii) Our
evaluation of the full reptation model yields an excellent fit to the data for
both total chains and internal pieces and for all wave vectors and all times,
provided specific micro-structure effects of the MC-dynamics are negligible.
Such micro-structure effects show up for wave vectors of the order of the
inverse segment size. For the dynamics of the total chain our data analysis
based on the full reptation model shows the importance of tube length
fluctuations. Universal (Rouse-type) internal relaxation is unimportant. It can
be observed only in the form of the diffusive motion of a short central
subchain in the tube. -- Finally we present a fit formula which in a large
range of wave vectors and chain lengths reproduces the numerical results of our
theory for the scattering from the total chain.
