Giovani: Which compiler are you using?
Also, you didn't mention this, but does "mpirun hostname" give the expected response? I (also new) had a hang similar to what you are describing due to ompi getting confused as to which of two network interfaces to use - "mpirun hostname" would hang when started on certain nodes. This problem was resolved by telling ompi which network interface to use (I forget the option needed to do this off the top of my head, but it is in the FAQ somewhere). Good luck, Mark
