Estimating molecular shapes

It calculates the hydrodynamic size of the diffusing species using three models: sphere, cylinder or ellipsoid, providing Ro or the semi-axis dimensions for a given P (a/b) or Q (b/a) geometry. The Ro parameter is usually interpreted as the average hydrodynamic radius of a solid sphere having the same volume of the calculated cylinder or ellipsoid.

$$D = \frac{k·T}{c(r_s,R_H)· f_s(a,b) · \pi · \nu · R_H}$$
$$c(r_s,R_H) = 6 · f(r_{sl},R_H)$$
$$\frac{\partial \psi(t,z)}{\partial t} = -i · \gamma · G(t,z) · \psi(t,z) + D · \nabla^2 \psi(t,z) - v_z · \nabla \psi(t,z)$$