This modifier determines the number of neighbors of each particle which are within
a given cutoff radius. This is the so-called coordination number of a particle.
The modifier stores the results in the
Coordination particle property.
In addition, the modifier also computes the radial pair distribution function (RDF) of the particle system. The RDF g(r) measures the probability of finding a particle at distance r given that there is a particle at position 0; it is essentially a histogram of inter-particle distances. The pair distribution function is normalized by the number density of particles (i.e. total number of particles divided by the simulation cell volume).
The current version of the modifier can only calculate the global RDF ignoring the types of particles. As explained in this forum post, you can also compute the partial RDFs for a system containing more than one particle species using a different approach.