            used for the 2nd-order force constant calculation</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>
        <span class="k">if</span> <span class="n">ngrid</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="n">ngrid</span> <span class="o">=</span> <span class="p">[</span><span class="mi