where g t o t a l ( r ) is the radial distribution function, i.e. g ( r ) = exp ⁡ ⁡ [ − − β β w ( r ) ] (with w (r) the potential of mean force) and g i n d i r e c t ( r ) is the radial distribution function without the direct interaction between pairs u ( r ) included; i.e. we write g i n d i r e c t ( r ) = exp ⁡ ⁡ [ − − β β ( w ( r ) − − u ( r ) ) ] . Thus we approximate c (r) by