The computer simulation of stress relaxation in a model polymer melt is performed with a nonequilibrium molecular dynamics algorithm. The chains are freely jointed with N = 30, 100, and 200 bonds and with both intra- and interchain excluded volume interactions. For N = 200, the model exhibits incipient plateau behavior as evidenced by an inflection point in the stress relaxation history. Comparison is made between the stress history as computed on the atomic level by the virial stress formula and on the molecular level using the entropic spring formulation. The two histories are in agreement only for the case of N = 200, with entanglement length N-e = 40, and only for the time period following the inflection point.