$U_S\left(x\right)=-k_{B}T\ln \left[{\left(\frac{2\pi n_C{b}^2}{3}\right)}^{-\frac{3}{2}}\left[\exp \left(\frac{-3{\left(l_t-\delta \right)}^2}{2n_C{b}^2}\right)-\exp \left(\frac{-3{\left(l_t+\delta \right)}^2}{2n_C{b}^2}\right)\right]\mathrm{erf}\left(\sqrt{\frac{3}{2n_T}}\frac{\delta }{b}\right)\right].$

(5)