lass="sd">        .. math::</span>

<span class="sd">            A_{ij} = \\frac{1}{\lvert \mathbf{r}_i - \mathbf{r}_j \\rvert }</span>

<span class="sd">        For periodic systems the distance of an atom from itself is the</span>
<span class="sd">        smallest displacement of an atom from one of it&#39;s periodic copies, and</span>
<span class="sd">        the distance of two different atoms is the distance of two closest</span>
<span class="sd">        copies.</span>

<span class="sd">        Returns