----- Original Message ----- From: Vitaly Chaban <[email protected]> Date: Wednesday, November 3, 2010 6:10 Subject: [gmx-users] pressure of the liquid in NPT and NVT To: [email protected]
> 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. Unless the box was equilibrated at NPT at the start of the NPT run, or the run was very long, the average value over the whole run is not all that likely to be one from the equilibrium range. Take the last frame. > 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? We had a thread about this last week, and numerous more in the past. These observations are strongly dependent on the length of time over which you observe the average pressure, and the size of the system. Note that your fluctuations dwarf your average... See http://www.gromacs.org/Documentation/Terminology/Pressure_Coupling Mark
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