Type Strain $\Delta E/{V}_{0}$ (0) isotropic ${e}_{1}=\delta$ ${e}_{2}=\delta$ ${e}_{3}=\delta$ (see bulk modulus) (1) monoclinic ${e}_{1}={\delta }^{2}/\left(1-{\delta }^{2}\right)$ ${e}_{4}=\delta$ $2{c}_{44}{\delta }^{2}+O\left({\delta }^{4}\right)$ (2) triclinic ${e}_{3}={\delta }^{2}/\left(1-{\delta }^{2}\right)$ ${e}_{6}=\delta$ $2{c}_{66}{\delta }^{2}+O\left({\delta }^{4}\right)$ (3) orthorhombic ${e}_{1}=\delta$ ${e}_{2}=-\delta$ ${e}_{3}={\delta }^{2}/\left(1-{\delta }^{2}\right)$ $\left({c}_{11}-{c}_{12}\right){\delta }^{2}+O\left({\delta }^{4}\right)$ (4) orthorhombic ${e}_{1}=\delta$ ${e}_{2}={\delta }^{2}/\left(1-{\delta }^{2}\right)$ ${e}_{3}=-\delta$ $\left({c}_{11}-2{c}_{13}+{c}_{33}\right){\delta }^{2}/2+O\left({\delta }^{4}\right)$ (5) tetragonal ${e}_{1}=\delta$ ${e}_{2}=\delta$ ${e}_{3}=-\delta \left(2+\delta \right)/{\left(1+\delta \right)}^{2}$ $\left({c}_{11}+{c}_{12}+2{c}_{33}-4{c}_{13}\right){\delta }^{2}/2+O\left({\delta }^{4}\right)$ (6) tetragonal ${e}_{3}=\delta$ ${c}_{33}{\delta }^{2}/2$