The background I b (s) in the simplest case can be taken as a pure atomic scattering intensity. In more complicated cases, with known or unknown systematic contributions, it can be extracted using various algorithms discussed in the literature. For a semi-rigid molecule, i.e., molecule without the largeamplitude motions, the sM(s) can be represented as sM(s) = ∑ p,q<p g pq (s) r e,pq