We have studied the chain dynamics of poly(ethylene-alt-propylene) by combining neutron spin echo experiments and fully atomistic molecular dynamics simulations. We have focused on molecular weights of the order of two times the entanglement mass, for which, in principle, the topological constraints are not important and the chain dynamics should be well described in terms of the Rouse model. From the comparison between experimental and simulated results we have validated our simulation cell. We have checked the predictions of the Rouse model on both sets of data and compared the resulting relevant parameters with those obtained from other experimental works in the literature. Moreover, taking advantage of the validated simulated system we have calculated directly the Rouse modes and correlators to investigate the limitations of the model extending the study at lower temperatures. We have found deviations from Rouse behavior at short length scales that can be attributed to the local potentials and a sublinear increase of the mean squared displacement of the chain center of mass with time, probably originating from intermolecular interactions.