3.13: F(000)
The expression for a structure factor evaluated in the zeroth-order case h = k = l = 0 yields the result
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 \(\theta = 0^\circ\), θ 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.