  We propose a new method for carrying out field-theoretic simulations of
polymer systems under conditions of prescribed external stress, allowing for
shape changes in the simulation box. A compact expression for the deviatoric
stress tensor is derived in terms of the chain propagator, and is used to
monitor changes in the box shape according to a simple relaxation scheme. The
method allows fully relaxed, stress free configurations to be obtained even in
non trivial morphologies, and enables the study of morphology transitions
induced by external stresses.
