Nonlinear response for nonautonomous systems

We present a detailed derivation of the transient time correlation function (TTCF) form of nonlinear response theory which is generalized to handle time-dependent external fields. Our derivation stresses the analogy with the TTCF formalism for constant fields. We also discuss some limiting cases. We use computer simulation to test the generalized TTCF theory. Our simulation results show that the generalized TTCF method has an efficiency superior to direct calculation not only for weak fields but also for strong time-dependent external fields.