| 11:20:56 AM PST - Tue, Dec 4th 2012 |
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The pressure can certainly be negative, in particular at the start of an MD simulation of a non-equilibrated system. This indicates that the system is contracting. Once equilibrated, the instantaneous pressure can still be negative, but the average over time should converge to the value of the external pressure as set in the input.
The statistics of properties are printed on the output file for the last 'nstat' MD steps as well as for the entire simulation. These are the two sets of three columns you see. Obviously the very first time that this block is written, the two sets are identical because the entire simulation has only progressed 'nstat' steps.
Continue the simulation for a resonable simulation time and both issues should be resolved.
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