In addition to being easy to locate from a Patterson vector map, the heavy atom makes a major contribution to the structure factors:

and provides a means to calculate an approximation to the phase angle:


A Fourier electron density map can now be calculated, using the phase angle determined by the location of the heavy atom and the experimentally determined structure factor amplitudes:


If the “heavy atom approximation” is good enough, this Fourier map will have peaks corresonding to the locations of the other atoms in the cell.