The Radial Distribution Function, R.D.F. , g(r), also called pair distribution function or pair correlation function, is an important structural characteristic, therefore computed by I.S.A.A.C.S..
Considering a homogeneous distribution of the atoms/molecules in space, the g(r) represents the probability to find an atom in a shell dr at the distance r of another atom chosen as a reference point [Fig. 1].
By dividing the physical space/model volume into shells dr [Fig. 1] it is possible to compute the number of atoms dn(r) at a distance between r and r + dr from a given atom:
where N represents the total number of atoms, V the model volume and where g(r) is the radial distribution function.
In this notation the volume of the shell of thickness dr is approximated V_{shell} = π(r + dr)^{3}  πr^{3} 4π r^{2} dr.
When more than one chemical species are present the socalled partial radial distribution functions g_{αβ}(r) may be computed :
where c_{α} represents the concentration of atomic species α.
These functions give the density probability for an atom of the α species to have a neighbor of the β species at a given distance r. The example features GeS_{2}glass.
Figure [Fig 2] shows the partial radial distribution functions for GeS_{2}glass at 300 K. The total RDF of a system is a weighterd sum of the respective partial RDFs, with the weights depend on the relative concentration and xray/neutron scattering amplitudes of the chemical species involved.
I.S.A.A.C.S. gives access to the partial distribution functions g_{αβ}(r) as well as to the partial reduced distribution functions G_{αβ}(r) defined by:
Two methods are available to compute the radial distribution functions:
 The standard real space calculation typical to analyze 3dimensional models
 The experimentlike calculation using the Fourier transform of the structure factor obtained using the Debye equation
