[OMPI users] Problem submitting TORQUE jobs with ppn=1

[Gijsbert Wiesenekker]

I have a four-node quad core cluster. I am running OpenMPI (version 1.4.2) jobs 
with Torque (version 2.4.8). I can submit jobs using
#PBS -lnodes=4:ppn=4
And 16 processes are launched. However if I use
#PBS -lnodes=4:ppn=1 
or
#PBS -lnodes=4
The call to MPI_Init is succesful, but the call to 
MPI_Comm_size(MPI_COMM_WORLD, &mpi_nprocs)
hangs and never returns.

Any ideas? Any workarounds?

Gijsbert