Rosen and Morse potential

The Rosen and Morse potential is given by (Eq. 1 ):

$\mathrm{\Phi }(r)=B\tanh (r/d)-C{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{h}}^2(r/d)$

the minimum is located at

$r_{\mathrm{m}\mathrm{i}\mathrm{n}}=-{\tanh }^{-1}(B/2C)$

as long as $\left|B\right|<2C$.