Parallel execution takes longer than serial


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timing information
Under parallel execution, the cpu time decreases by a factor of (1/3), but the wall time doubles compared to serial execution:

Parallel Timing:

    >>>  JOB COMPLETED     AT Sat Apr  7 07:46:57 2012  <<<

Task  times  cpu:       48.1s     wall:      178.5s
Summary of allocated global arrays


 No active global arrays



                        GA Statistics for process    0
------------------------------

      create   destroy   get      put      acc     scatter   gather  read&inc
calls: 0 0 0 0 0 0 0 0
number of processes/call 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
bytes total: 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
bytes remote: 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
Max memory consumed for GA by this process: 0 bytes
MA_summarize_allocated_blocks: starting scan ...
MA_summarize_allocated_blocks: scan completed: 0 heap blocks, 0 stack blocks
MA usage statistics:

allocation statistics:
heap stack
---- -----
current number of blocks 0 0
maximum number of blocks 292 19
current total bytes 0 0
maximum total bytes 160737864 32171064
maximum total K-bytes 160738 32172
maximum total M-bytes 161 33


                               NWChem Input Module
-------------------

Total times cpu: 56.9s wall: 317.7s
Creating: host=dirac.asbury.edu, user=deburg0,
file=/scratch/deburg0/nwchem-6.1/bin/LINUX64/nwchem, port=49546




Serial Timing:
    >>>  JOB COMPLETED     AT Sat Apr  7 08:11:59 2012  <<<

Task  times  cpu:      151.1s     wall:      151.3s
Summary of allocated global arrays


 No active global arrays



                        GA Statistics for process    0
------------------------------

      create   destroy   get      put      acc     scatter   gather  read&inc
calls: 0 0 0 0 0 0 0 0
number of processes/call 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
bytes total: 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
bytes remote: 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00 0.00e+00
Max memory consumed for GA by this process: 0 bytes
MA_summarize_allocated_blocks: starting scan ...
MA_summarize_allocated_blocks: scan completed: 0 heap blocks, 0 stack blocks
MA usage statistics:

allocation statistics:
heap stack
---- -----
current number of blocks 0 0
maximum number of blocks 292 19
current total bytes 0 0
maximum total bytes 638357352 128682232
maximum total K-bytes 638358 128683
maximum total M-bytes 639 129


                               NWChem Input Module
-------------------
Total times cpu: 179.2s wall: 179.6s