There is a simple test to check if the parallelization is correctly set up. Enter the following into a script
import socket
if processIsMaster():
print 'Master node:',
else:
print 'Slave node:',
print socket.gethostname()
and execute it in parallel. Make sure to capture the output.
It should print "Master" once and "Slave" N-1 times, where N is your -n N in mpiexec. This will be the signal that you have a proper MPI setup.
For OpenMPI and its relatives it prints "Master" N times, however, and that tells you that all processes think they are masters, and will try to write to the NetCDF file, and this of course causes problems.
For more information about running ATK in parallel, see the Parallel Tutorial (http://quantumwise.com/documents/tutorials/latest/ParallelGuide/).