[AMBER] sander.MPI is running on 8 nodes, but the output says it is running on 1 nodes

From: M Kang <medichem.gmail.com> Date: Fri, 22 Jul 2011 11:36:49 -0700

----
#!/bin/sh
INPUT=sander.in
cat $PBS_NODEFILE
/opt/amber11-ambertools-1.4/
bin/lamboot -d -v  $PBS_NODEFILE
echo "mpirun" 1>&2
/opt/amber11-ambertools-1.4/bin/mpirun -np 8
/home_changlab/croberts/local/amber11/bin/sander.MPI  -O `cat sa
nder.in`
echo "lamclean" 1>&2
/opt/amber11-ambertools-1.4/bin/lamclean
echo "lamhalt" 1>&2
/opt/amber11-ambertools-1.4/bin/lamhalt
-----
Then I run "top" command, which shows 8 processors are running sander.MPI
about 100%.
But the output says it is running on 1 nodes.
---
|  Atom division among processors:
|         0    3128
|  Running AMBER/MPI version on    1 nodes
---
And actually it is more than 3 times slower than a sander.MPI (amber10) job
with 4 nodes in the other machine.
---
|  Atom division among processors:
|         0     797    1564    2361    3128
|  Running AMBER/MPI version on    4 nodes
---
It doesn't show any error message. How can I fix this problem?
*Waiting for some suggestion in this regards.
Myung*
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