Above, − − ∇ ∇ j w ( n ) is the averaged force, i.e. "mean force" on particle j. And w ( n ) is the so-called potential of mean force. For n = 2 , w ( 2 ) ( r ) is the average work needed to bring the two particles from infinite separation to a distance r . It is also related to the radial distribution function of the system, g ( r ) , by: