SUR is the simplest symplectic propagator applicable to quantum wavepacket propagation in the energy representation. We decompose the algorithm into unimodular modules that allow the implementation of complex, time-varying matrix elements. This allows the algorithm to be applied to the propagation of wavepackets coupled to a time-varying classical external field, without changing the scaling of the computational time with system size. An adaptive time step version of the algorithm is implemented. (c) 2006 Elsevier B.V. All rights reserved.