A cold atoms gravimeter il a device which measure the gravity with Raman transitions on free falling atoms. Since 2006, the SYRTE research team is working on such a gravimeter, and during my thesis, I will continue their work. Today, this gravimeter is working with an accuracy of 5.7 10^-8. The limits on this result are known: the wavefront aberration. The combination of the velocity distribution of the atoms and the irregularity of the optics create a complicated to evaluate bias on g which fluctuate in time. This cause a degradation of the accuracy and the long term stability. In this talk, I will present the work I did during my internship, just before the start of my thesis. With the form of the optics and the incoming wavefront as input, it may be possible to simulate the working of the gravimeter, and thus deduce a numerical value of this bias. To do this, I searched a way to propagate any wavefront in free space.