3.11: Electron density map
A three-dimensional description of the electron density in a crystal structure, determined from X-ray diffraction experiments. X-rays scatter from the electron clouds of atoms in the crystal lattice; the diffracted waves from scattering planes h , k , l are described by structure factors \(\mathbf{F}_{hkl}.\) The electron density as a function of position x , y , z is the Fourier transform of the structure factors:
\(\rho(xyz) = {1\over V}\sum_{hkl} F(hkl)\exp[-2\pi i(hx + ky + lz)]\).
The electron density map describes the contents of the unit cells averaged over the whole crystal and not the contents of a single unit cell (a distinction that is important where structural disorder is present).
Three-dimensional maps are often evaluated as parallel two-dimensional contoured sections at different heights in the unit cell.
Units
Electron density is measured in electrons per cubic ångström, e Å -3 .