span>
<span class="sd">            np.array: Symmetric 2D matrix containing the pairwise distances.</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">_distance_matrix</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">get_displacement_tensor</span><span class="p">()</span>
        <spa