The program Fourier calculates the scattering density inside the unit cell of a crystal of whatever symmetry. It uses a Fast Fourier Transform (FFT) subroutine to accelerate the calculation of the following expression:
where V is the volume of the unit cell. H is a reciprocal lattice vector, r is a vector position inside the unit cell, and F(H) are complex Fourier coefficients used to perform different types of Fourier syntheses. The units of ρ(r) are those of F(H) divided by those of V. For instance, if F(H) are given en electron units (usual absolute units for X-ray diffraction) and V in Å3, ρ(r) is calculated as Number-of-Electrons / Å3. In order to get absolute values of the density, the value of F(000) should be provided (see F000 keyword).