Nematic twist ($K_2(\mathbf{n}\cdot \nabla \times \mathbf{n}{)}^2/2$)

Nematic twist rotates the nematic director around an axis perpendicular to it. The helix $\mathbf{n}=(\cos qz,\sin qz,0)$ has zero nematic splay and nematic bend, and twist density $K_2{q}^2/2$.