Yukawa closure

The Yukawa closure for hard spheres is

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

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle c_{ij}(r)= \sum_{n=1} \frac{K_{ij}}{r}e^{-z_nr} ~~~~~~ \sigma_{ij} < r }