3.11: Electron density map

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Page ID: 17703

Online Dictionary of Crystallography
International Union of Crystallography

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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.

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Electron density is measured in electrons per cubic ångström, e Å^-3.

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