Hi, I also met the same problem when I use the fix msd command. The program finished successfully, and the msd output file is created, but nothing inside. I also tried to use dump custom to output the result, but it doesn’t work. The command used: “fix 2 water msd 5000 watermsd.out”
After read the reply from steve, I changed the fix_msd.cpp and make lammps again, now it works well 
Best Regards,
tony