Figure : Illustration of the central concepts of volume and densities in a finite (periodic) system with volume V cell . Particle l takes up space in the simulation cell, where the other particles cannot exist. This volume is dependent on the extension of both of the particle types, as the volume is defined by the interaction potential between the particles. Thus, for a potential of non-interacting hard spheres, the volume would be a sphere with radius R lm = R l + R m , but for all other differentiable potentials, the volume will be less well defined. When scaling the RDF to ρ m , the excluded volume of particle type-pair l, m, V lm , is included in V cell , even though it is not available to the m-type particles. Instead, the "effective volume", i.e. the total volume of the simulation cell minus V lm should be used to obtain an "effective" density ρ m,eff with which ρ(r) in eq. 1 should be normalized.