F(000)

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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 fr is the real part of the scattering factors at \theta = 0^\circ, fi 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.