[gmx-users] question: negative pressure

[lugalzagissi]

Hello.

My problem is the next:
I ran the 5 ns simulation of a molecule of polyethylene C1000 with pressure coupling under the temperature of 1000K. The parameters for p.coupling are the next:

Pcoupl                   =  berendsen
Pcoupltype               = isotropic
tau_p                    = 1.0
compressibility          = 4.5e-5
ref_p                    = 1.0

The average pressure for all the simulation is negative. These results I've got with g_energy:

Energy                      Average       RMSD     Fluct.      Drift  Tot-Drift
-------------------------------------------------------------------------------
Potential                   25695.5    347.159    341.527   0.117271    215.748
Kinetic En.                 24928.9    294.821    294.818 0.00230749    4.24516
Total Energy                50624.4    184.928    173.682   0.119579    219.993
Temperature                  999.58    11.8215    11.8214 9.2524e-05   0.170219
Pressure (bar)           -0.0807379    41.7124    41.7124 1.04028e-05  0.0191384
Volume                      946.272    29.3151    1.62577 -0.0551136   -101.394

Could anybody explain, why?

Thanks