sorry my english..

I want to know how can I run  Gromancs in parallel!
Because when I used 

mdrun &
mpiexec -np 4 mdrun_mpi -v -deffnm em

 to run the minimization in 4 cores > all cores make the same job, again!
They don't run together. 
I want all in parallel make the job faster.

what could be wrong?

thank's a lot!