Hello Hossein,
There are differences in the SMP and MPP implementations of LS-DYNA and so running the same .k file with SMP might give slighthly different results compared with running MPP. The main difference between SMP and MPP is in the contact algorithm implementation.
I would recommend that you run MPP on your personal computer as well to see if you get the negative volume error. If so, you can then debug this problem.
Also, it is recommended to always run with the same number of MPP cores. This is because the domain of your model will be decomposed (or split) into n number of cores when running MPP. When using different number of cores, the decomposition will be different and the results will change slightly. If your model is close to instability, changing the number of MPP cores may be enough to get the negative volume error. The negative volume error is an indication of instabilities.
You will find more information about MPP here:
http://ftp.lstc.com/anonymous/outgoing/support/PRESENTATIONS/mpp_201305.pdf
Reno.
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