F(000)

The expression for a structure factor evaluated in the zeroth-order case $h=k=l=0$ yields the result

$F(000) = [ (\sum f_{r} )^{\,2} + (\sum f_{i} )^{\,2} ]^{1/2}$

where $f_{r}$ is the real part of the scattering factors at $\theta = 0^\circ$, $f_{i}$ is the imaginary part of the scattering factors at $\theta = 0^\circ$, $\theta$ is the Bragg angle, and the sum is taken over each atom in the unit cell.

$F(000)$ is computed without dispersion effects in electron-density calculation by Fourier inversion. In all cases, non-dispersive $F(000)$ is a structure factor and not a structure amplitude: it has both magnitude and a sign.

For X-rays non-dispersive $F(000)$ is positive definite and in many cases an integer (but it is not an integer for non-stoichiometric compounds). It counts the number of electrons in the cell.

For neutrons non-dispersive $F(000)$ is either positive or negative and counts the total nuclear scattering power in the cell.