The displacement of a particular atom in a modulated crystallographic structure can be described in terms of a Fourier Series. In the harmonic single wave vector approximation, the structure factor can be written as a function of the atomic displacement vectors. Such expressions are however not valid in the multi-wave-vector case. We propose a general description for the modulation of displacement where each scattering center (atom) is distributed on several sites whose population is modulated. In the paper, we will show that approach is well suited for computing. We have developed a general least squares program which has been applied to the modulated phase of αU(T < 43 K) [2].