Renormalization-group approach to nonequilibrium Green functions in correlated impurity systems

We present a technique for calculating nonequilibrium 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 nonequilibrium dynamics of the Ohmic two-state problem.