  Using molecular dynamics simulations, we determine the linear and nonlinear
viscoelastic properties of a model polymer melt in the unentangled regime.
Several approaches are compared for the computation of linear moduli, including
Green-Kubo and non equilibrium molecular dynamics (NEMD). An alternative
approach, based on the use of the Rouse modes, is also discussed. This approach
could be used to assess local viscoelastic properties in inhomogeneous systems.
We also focus on the contributions of different interactions to the
viscoelastic moduli and explain the microscopic mechanisms involved in the
mechanical response of the melt to external sollicitation.
