  Following the Flory ideality hypothesis intrachain and interchain excluded
volume interactions are supposed to compensate each other in dense polymer
systems. Multi-chain effects should thus be neglected and polymer conformations
may be understood from simple phantom chain models. Here we provide evidence
against this phantom chain, mean-field picture. We analyze numerically and
theoretically the static correlation function of the Rouse modes. Our numerical
results are obtained from computer simulations of two coarse-grained polymer
models for which the strength of the monomer repulsion can be varied, from full
excluded volume (`hard monomers') to no excluded volume (`phantom chains'). For
nonvanishing excluded volume we find the simulated correlation function of the
Rouse modes to deviate markedly from the predictions of phantom chain models.
This demonstrates that there are nonnegligible correlations along the chains in
a melt. These correlations can be taken into account by perturbation theory.
Our simulation results are in good agreement with these new theoretical
predictions.
