[Pw_forum] neb

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Hi 
In order to test how much time can parallel cluster take time when working on neb job and comparing it with the time that can one CPU take  ,we put  neb job with 15 atom on
 1-a cluster (MPICH2) of 4 nodes (2 CPU in one station ).
 2- a cluster (MPICH2) OF 2 nodes (2 CPU in one station )
3- On one CPU 
we used for 1 &2 
mpiexec -n 4 pw.x <-->--

mpiexec -n 2 pw.x <-->--


All CPU have same properties (8G for Ram ),opteron AMD.
We found that the job on one CPU (3rd one )took less time than others.!!!
Why this happen could anybody tell us please?
best regards 
Ihsan Erikat
Jordan -Physics department