At the moment, I'm installing the "GADGET-2" application. In its documentation it says "Note that the MPI-capable version 2.x of FFTW is required, the new version 3 lacks MPI capability at this point. FFTW is only needed for simulations that use the TreePM algorithm."
Is this still true for the current version 3.3.3? Should I use 2.1.5 or 3.3.3?