Yukawa closure

The Yukawa closure relation for hard spheres is

$g_{ij}(r)\equiv h_{ij}(r)+1=0~~~~~~~~r<\sigma _{ij}=(\sigma _{i}+\sigma _{j})/2$

and

$c_{ij}(r)=\sum _{n=1}{\frac {K_{ij}}{r}}e^{-z_{n}r}~~~~~~\sigma _{ij}<r$