n>
    <span class="n">n_atoms</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">struct</span><span class="o">.</span><span class="n">species</span><span class="p">)</span>
    <span class="k">if</span> <span class="n">n_atoms</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
        <span class="k">return</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">