If the system consists of spherically symmetric particles, g ( 2 ) ( r 1 , r 2 ) depends only on the relative distance between them, r 12 = r 2 − − r 1 . We will drop the sub- and superscript: g ( r ) ≡ ≡ g ( 2 ) ( r 12 ) . Taking particle 0 as fixed at the origin of the coordinates, ρ ρ g ( r ) d 3 r = d n ( r ) is the average number of particles (among the remaining N − − 1 ) to be found in the volume d 3 r around the position r .