\begin{equation}
U_{\rm el,pp} =
\frac {e^2}{8\pi\epsilon_{0}\epsilon_{\rm r}}
{\sum_{\mu,\nu=1}^{n_{p}}\sum_{i,j =1}^{N}}
\Bigl(1 - \delta_{\mu\nu}\delta_{ij}\Bigr)
\frac {\sigma_i\sigma_j}{r_{{\mu}i,{\nu}j}}
\; ,
\end{equation}
