Rotational diffusion ($D_r$)

Rotational diffusion randomizes orientation. For a unit orientation vector on a sphere, isotropic rotational diffusion contributes $D_r{\nabla }_p^{2}f$ to the Fokker-Planck equation, where ${\nabla }_p^2$ is the Laplace-Beltrami operator. Its coefficient has units inverse time. It differs from the translational diffusion coefficient, whose units are length squared per time.