 of unique atomic numbers, a set</span>
<span class="sd">        of atomic numbers and the original variable contents.</span>

<span class="sd">        Args:</span>
<span class="sd">            species(iterable): Chemical species either as a list of atomic</span>
<span class="sd">                numbers or list of chemical symbols.</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="c1"># The species are stored as atomic numbers for internal use.</span>
        <span class="n">ato