Yoshida and Kamakura model

The Yoshida and Kamakura model is given by (Eq. 1 in ):

$\Phi _{12}(r)=\epsilon \exp \left(a\left(1-{\frac {r}{\sigma }}\right)-6\left(1-{\frac {r}{\sigma }}\right)^{2}\ln \left({\frac {r}{\sigma }}\right)\right)$