I am now doing some research about DM halo angular momentum distribution using Illustris Simulation data. However I meet some problem when I read out the data. According to the website documents, I know the FoF halo catalog provide the Group_M_TopHap200 and Group_R_TopHat200 values for each fof halo. Also I can use "il.snapshot.loadHalo" commend to read all the particles belong to this FoF halo. But with all these particles information, I can only calculate the center of mass of this FoF halo. The FoF halo catalog does not provided the halo center coordinates which is used to calculate Group_M_TopHap200 and Group_R_TopHat200 values. In fact, I need to use those center coordinates (not CM center) to calculate the spin parameter λ .
The halo center position we typically use is available in GroupPos, which is "the location of the particle with the minimum gravitational potential energy". I think this should work?