Experimental 2D frequency resolved spectra showing the relaxation of excitons in the photosynthetic antenna system FMO are reproduced by a multi-exciton density matrix approach explicitly including the external fields. The interference of the polarizations of a number of randomly placed, rotated and energetically inhomogencous complexes is used to extract the signal in the phase matching directions. It is shown that varying the laser intensity has only a minor impact, but changing the phase within the single pulses makes higher order contributions possible which lead to a fundamental change of the 2D spectra. (C) 2007 Elsevier B.V. All rights reserved.