  We present a technique for calculating non-equilibrium Green functions for
impurity systems with local interactions. We use an analogy to the calculation
of response functions in the x-ray problem.The initial state and the final
state problems, which correspond to the situations before and after the
disturbance (an electric or magnetic field, for example) is suddenly switched
on, are solved with the aid of Wilson's momentum shell renormalization group.
The method is illustrated by calculating the non-equilibrium dynamics of the
ohmic two-state problem.
