RMV, thanks again for your support and Fortran code (which I have successfully ported to python with the help of the cited papers [Fortran code is not that straight forward when you are used to python :-) ]).
Still I am wondering about one thing. The long time diffusion should be calculated from a region where the r2com is more or less linear.
From the attached picture (r2com for 86 POPC) I would tend to use frames between ~6000 and ~13000.
Is this reasonable? I am somewhat unsure if it is okay to discard so much points.
Currently I am calculating r2com for different time windows to get a clue how this effects the shape of the curve.