[gmx-users] pressure of the liquid in NPT and NVT

[Vitaly Chaban]

Hi all,

Please suggest me what is the reason for the below situation.

The 1728 TIP4P waters system was simulated in NPT under 1bar
maintained with Parrinello-Rahman barostat at 300K. Some results are
below:

Energy                      Average       RMSD     Fluct.      Drift  Tot-Drift
-------------------------------------------------------------------------------
Total Energy               -58121.6    344.637    344.476   0.182549    36.5101
Temperature                 299.913    4.11684     4.1138 0.00274337   0.548679
Pressure (bar)              1.39811    326.506    317.633   -1.30948     -261.9
Box-X                       3.74431 0.00844567 0.00843154 -8.45584e-06
-0.00169118
Density (SI)                984.768    6.66431    6.65307  0.0067005    1.34011

==========================================================

The density is stable and the system looks like equilibrated one.

The output GRO was then renamed to conf.gro and the average Box-X from
the previous run was set as the cubic box side length. The barostat
was switched off with no other changes in MDP. The same properties for
NVT are below:

Energy                      Average       RMSD     Fluct.      Drift  Tot-Drift
-------------------------------------------------------------------------------
Total Energy               -58075.8    309.575    308.343  -0.477939   -95.5887
Temperature                 299.916    4.08901    4.08678 -0.00233712  -0.467428
Pressure (bar)             -18.5469    286.376    285.574    0.37081    74.1628


So the pressure in NPT is ~1.4 and in NVT is ~(-19). How to explain so
big difference in average values? Is it possible to get a value of
around 1 in the NVT?

Thank you.
Vitaly