F(000)

The expression for a structure factor evaluated in the zeroth-order case [math]\textstyle{ h=k=l=0 }[/math] yields the result

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

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

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

For X-rays non-dispersive [math]\textstyle{ F(000) }[/math] 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 [math]\textstyle{ F(000) }[/math] is either positive or negative and counts the total nuclear scattering power in the cell.