Thanks for the quick reply.
Openmpi version is 1.6.5 on both machines. For the iMac I use the default version and configurations provided with ALPS, while for Ubuntu I use the version and configurations in the Ubuntu repositories. What specific configuration should I look for?
Running an example C++ program also gets into trouble. For instance, from the iMac I get normal execution with 'mpirun -H localhost a.out' as well as with 'mpirun -H linuxhost a.out', but when I try 'mpirun -H localhost,linuxhost a.out' I get the following (dirac-2 is the hostname for the iMac)
[dirac-2.local:22207] *** An error occurred in MPI_Recv
[dirac-2.local:22207] *** on communicator MPI_COMM_WORLD
[dirac-2.local:22207] *** MPI_ERR_TRUNCATE: message truncated
[dirac-2.local:22207] *** MPI_ERRORS_ARE_FATAL: your MPI job will now abort
--------------------------------------------------------------------------
mpirun has exited due to process rank 1 with PID 22207 on
node dirac-2 exiting improperly. There are two reasons this could occur:
1. this process did not call "init" before exiting, but others in
the job did. This can cause a job to hang indefinitely while it waits
for all processes to call "init". By rule, if one process calls "init",
then ALL processes must call "init" prior to termination.
2. this process called "init", but exited without calling "finalize".
By rule, all processes that call "init" MUST call "finalize" prior to
exiting or it will be considered an "abnormal termination"
This may have caused other processes in the application to be
terminated by signals sent by mpirun (as reported here).
Running from the linux host generates the same behavior, with the same error message, so I guess there is something in the iMac (dirac-2) preventing communication between hosts.