Frank elastic constants

Distortion free energy

$F_{d}={\frac {1}{2}}K_{1}({\rm {div}}~\mathbf {n} )^{2}+{\frac {1}{2}}K_{2}(\mathbf {n} \cdot {\rm {curl}}~\mathbf {n} )^{2}+{\frac {1}{2}}K_{3}(\mathbf {n} \times {\rm {curl}}~\mathbf {n} )^{2}$