To apply this theory to x-ray or neutron diffraction we go back to the Lippmann–Schwinger equation, | ψ ψ ⟩ = | ϕ ϕ ⟩ + G 0 + T | ϕ ϕ ⟩ = | ϕ ϕ ⟩ + ∑ ∑ j = 1 N | ϕ ϕ j o u t ⟩ . The scattering from a site is assumed to be very small, so | ϕ ϕ i o u t ⟩ = G 0 + t i | ϕ ϕ ⟩ or T = ∑ ∑ i = 1 N t i . The Born approximation is used to calculate the t-matrix, which simply means that t i is replaced with v i . A plane wave impinges on a site, and a spherical wave exits it. The outgoing wave from the crystal is determined by the constructive interference of the waves from the sites. Advances to this theory involve the inclusion of higher-order terms in the total scattering matrix T , such as ∑ ∑ i t i G 0 + ∑ ∑ j ≠ ≠ i t j . These terms are particularly important in the scattering of charged particles treated by Molière.