International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. B, ch. 1.3, p. 63

In many cases, is a sum of translates of atomic electrondensity distributions. Assume there are n distinct chemical types of atoms, with identical isotropic atoms of type j described by an electron distribution about their centre of mass. According to quantum mechanics each is a smooth rapidly decreasing function of x, i.e. , hence and (ignoring the effect of thermal agitation)which may be written (Section 1.3.2.5.8)By Fourier transformation:Defining the form factor of atom j as a function of h to bewe haveIf and are the real and reciprocalspace coordinates in Å and Å^{−1}, and if is the spherically symmetric electrondensity function for atom type j, then
More complex expansions are used for electrondensity studies (see Chapter 1.2 in this volume). Anisotropic Gaussian atoms may be dealt with through the formulae given in Section 1.3.2.4.4.2.