> ]$ f95 -O3 -lm -march=nocona -o nbody.x nbody.f90 > ]$ time ./nbody.x 50000000 > Energy 0: -0.169075164 > Energy 1: -0.169059907 > Elapsed time: 2m 31.7s > AFAIK, the GNU fortran compiler goes with the name gfortran (not f95). Please give the output of "gfortran -v" so that we will know the exact options you are using.