This modifier performs two things:
It determines the number of neighbors of each particle that are within
a given cutoff range, the so-called coordination number of a particle, and
stores this value in the
Coordination output particle property.
In addition, the modifier computes the radial pair distribution function (radial PDF, or simply RDF) for the entire particle system.
The radial pair distribution function g(r) measures the probability of finding a particle at distance r
given that there is a particle at position r=0; it is essentially a histogram of inter-particle distances. The pair distribution function is
normalized by the average number density of particles (i.e. the total number of particles in the simulation cell divided by its volume).
See the Wikipedia for more information on this distribution function.
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, more elaborate approach.
In the current program version, the Coordination Analysis modifier is restricted to the calculation of the instantaneous RDF for the current simulation frame only. If you want to calculate the RDF averaged over all frames of a simulation trajectory, you also need to use OVITO's scripting capabilities instead of the graphical user interface. See the corresponding section of the scripting manual for an example.