  We present a methodology for simulating multi-dimensional electronic spectra
of molecular aggregates with coupling of electronic excitation to a structured
environment using the stochastic non-Markovian quantum state diffusion (NMQSD)
method in combination with perturbation theory for the response functions. A
crucial aspect of our approach is that we propagate the NMQSD equation in a
doubled system Hilbert space, but with the same noise. We demonstrate that our
approach shows fast convergence with respect to the number of stochastic
trajectories, providing a promising technique for numerical calculation of
two-dimensional spectra of large molecular aggregates.
