where ρ m = N m /V is the average density of m-type particles in a simulation cell of volume V , and N lm (r) is the number of m-type atoms found at distance r + dr from atoms of type l, most often averaged over a set of MD frames. As we want to separate the various contributions to the total scattering signal, we introduce the indices w l , w m , since for a solute in a solvent, the atom types can be further classified as belonging to either the solute, w l = u, or the solv ent: w l = v. Then, the total scattered x-ray intensity I(q) as a function of the scattering vector q from a solute in a solvent can be calculated as the double sum of the individual terms (the form of which we shall return to in eq. 3):