The intensity is described as a function of the momentum transfer 𝑞 = | 𝐪 | = 4π (sin θ/λ), where 2θ is the scattering angle, λ the source wavelength and 𝑟 !" = |𝐫 !" | = |𝑟 ! -𝑟 " | is the distance from the atom i to the atom j, which represents the relative position of atoms i and j in the sample. The ⟨ … ⟩ notation refers to the spherical average, required to integrate the scattered intensity over all directions that have the same magnitude of q. In the case of SAXS, the radiation-matter interaction between the X-ray photon and the electron cloud of the atom i is described by the atomic scattering factor 𝑓 ! (𝑞), which can be approximated with the Cromer-Mann equation: