n><span class="n">def_grad</span><span class="p">)</span>
        <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">is_symmetric</span><span class="p">:</span>
            <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">&quot;The stress tensor is not symmetric, </span><span class="se">\</span>
<span class="s2">                             PK stress is based on a symmetric stress te