lass="k">else</span><span class="p">:</span>
        <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">&#39;symbols must have one or two values&#39;</span><span class="p">)</span>

    <span class="c1"># Initialize system (will shift second atom&#39;s position later...)</span>
    <span class="n">box</span> <span class="o">=</span> <span class="n">am</span><span class="o">.</span><span class="n">Box</span><span class="o">.</span><span class="n">cubic</sp