Abstract
The cumulant expansion method is used to compute quantities like 〈n(t) ⋅n(0) 〉, where n is a unit vector fixed on a Brownian particle immersed in a classical fluid, with an angular velocity obeying a generalized Langevin equation. The results are valid both in the limits of a free and of a completely hindered rotation. In the long time limit corrections to the Debye equation of diffusion on the unit sphere are found.