$$ \begin{array}{@{}rcl@{}} E(x) &=&\frac{1}{2}{\sum}_{i\in\mathcal{N}}E_{i}(x)\\ & =&\frac{1}{2}{\sum}_{i\in\mathcal{N}}{\sum}_{j\in\mathcal{R}_{i}}q_{i} q_{j}\left[ V(r_{ij})-V(r_{\text{c}})\right]\\ &-&\frac{V(r_{\text{c}})...