Dear Gromacs community, I modified the pull code in GROMACS to apply forces on atoms that are in a certain region of space. It compiles error free and runs fine to give me the expected results. I then recompiled with mpi support. The cluster I am using has 8 cores/processors per node. When I use 1, 2 or 4 nodes to run, I get identical results, but the results are very different when I run parallel on all 8 cores. 1. I am not sure how the forces I apply in the pull code are parallelized. 2. Is this easy for me to edit in the source code or is it best if I use only upto 4 processors for my job? 3. Will the number of processors that I am able to use change depending on the system size? I currently have on protein and about 7500 water molecules to give a total of about 25000 atoms. I am using SHAKE with constraints on h-bonds. Thanks for any help! Sumanth Sumanth N Jamadagni Graduate Student Isermann Dept of Chemical and Biological Engg Rensselaer Polytechnic Institute jamads@rpi.edu (Cell)518-598-2786 http://boyle.che.rpi.edu/~sumanth