  We present a theoretical justification for a method of extracting of
supplementary information for the phase retrieval procedure taken from
diffraction of fs-pulses from X-ray Free Electron Laser facilities. The
approach is based on numerical simulation of the dynamics of the electron
density in the crystal composed of different atoms in the unit cell, namely a
bi-atomic crystal containing heavy and light atoms. It is shown that evaluation
of diffraction intensities measured by means of different values of XFEL pulse
parameters enables to find absolute values of structure factors for both types
of atoms and their relative phase. The accuracy of structural information is
discussed in terms of fluctuations of the evaluated atomic scattering factors.
Our approach could be important for improvement of phase retrieval methods with
respect to a more efficient determination of atomic positions within the unit
cell of macromolecules.
